I just purchased a Dixon and love the tone! I find playing the whistle addictive, but am still having trouble with the lowest note (D). Any suggestions? Thanks.
The low register on most Dixons is difficult. Requires very little air. My suggestion: stick with it, or just buy a Feadog. 
Try playing down to the bottom D untill you find the amount of air it takes.
Most whistles take less for the bell note. By playing down to it you will be blowing softer when you get to it, and there will alwready be some air moveing through the tube.
My suggestion is the same as the others really. I changed whistles some time ago and I couldn’t get the bottom two notes at all. I was totally mystified.
And it did turn out that they needed much less air than I was accustomed to delivering. I also found that if I sort of shaped my mouth, cheeks, tongue, and throat in a way similar to what I would do if I was whistling a really low note just with my lips, that seemed to help. In other words, you can blow more gently and also change the shape of your mouth and this combination might help. I found that the higher notes on that whistle were easier to get than on my other whistle, so I don’t know if it is some sort of trade-off or not.
Welcome to C&F muk4821!
The Dixon whistles certainly have a nice tone! As everyone has suggested - stick with it.
If you are like many others here you will eventually progress to other instruments - each with its own challenges. It is this variety, that never seems to end, that makes whistles such an adventure! And as you progress you will appreciate the control that each whistle has taught.
As Congrats stated - the low notes require a lot of sensitivity on a Dixon - much less so on a Feadog - Feadogs have a great bottom-end, but require a lot of accuracy or else they squeak and squeal (even after a good tweaking).
After a while (and your second or third purchase of a high-end whistle) you may still find the Dixon has a part to play (quiet sessions, or practice scenarios - or just simply for the tone).
My susatos have a similar thing going. In the lower couple notes it is really easy to jump them to the second octave. Like everyone else the only real advice I can give is just play them down to that note until you find the right amount of air pressure to keep it in the octave you want. It will happen. 
I found the same problem with mine. I ended up learning to blow into it with very little lip around the mouth piece, allowing some air to escape on the sides of my mouth. Then as I play other notes I cover the mouthpiece more . Otherwise, as already mentioned, it requires very little air to begin with. If it was more of a whistle problem, Tony was more than happy to try to correct the problem, but I adjusted to playing it and knowing it was just not the whistle…it was me.
You have a fine whistle for learning, and all things considered, it’s very forgiving of beginner errors. Exept for blowing too much. Do what the others suggested – play down to that lower D from the G. And do this over and over again until you get a good feel for how little air it takes and how to set your mouth. There’s muscle memory involved, so the key si repetition.
I’m sitting around ar work with no adult supervision, so I’m fiddling with my Dixon to find out just what I do differently to get to that bottom D. I find that my mouth and cheek are pretty loose, although there is no air escaping around the edge. Tightening my lips a tad with no real increase in breath force jumps up to the middle D, and I can make that octave jumb back and forth with little effort and without even changing the fingering just by tightening my lips. ( I think I also pull my tongue back up a tad, but it’s mostly lips and cheeks.)
The muscles involved are the ones your wife or girlfriend uses when her lips disappear after you’ve pissed her off. Imagine this conversation –
“Honey, I’m leaving now.”
“But you promised to fix those shelves.”
“I know, but the guys need me because we don’t have a shortstop.”
“Fine. Go.”
And picture her face when she’s making that last statement. Those are the muscles that need to be loose for the low D and tight for the middle D.
See if you can have an experienced player try your Dixon. If he/she can get that note clearly, you’ll know it’s your technique that’s at fault. It might be possible to get a faulty Dixon revoiced to make that note easier, but doing so may sacrifice the higher notes to some extent. It’s a balancing act.
Thanks for all the great advice! It does seem to be getting easier and more consistent every day. It plays very nice, when I get it right, and seems to be a comination of the things that were mentioned. Thanks again!