I have an older Stilletto 100 and was wondering of it's possible to download music from iTunes to it.

You should have the MySiriusStudio software, if you don't, DL it from sirius.com
The MSS software is kinda like itunes for the stiletto. Import the songs to MSS, sync, profit.

I have the MySiriusStudio software installed on my PC. The MySiriusStudio application seems to only recognize mp3 and wma formats. From what I can tell iTunes uses m4a (whatever that is). Do I need to convert the m4a file to mp3 or wma? How do I do that? As you can tell I'm not very good with this kind of stuff. Thanks for your help.

I have the MySiriusStudio software installed on my PC. The MySiriusStudio application seems to only recognize mp3 and wma formats. From what I can tell iTunes uses m4a (whatever that is). Do I need to convert the m4a file to mp3 or wma? How do I do that? As you can tell I'm not very good with this kind of stuff. Thanks for your help.

No prob.
I don't know a lot, but lemme see if I can help.

In iTunes, you should be able to make .mp3 versions of tracks which are not.
Highlight the track(s) you want to convert to mp3
On the menu on top click Advanced
Click "Create MP3 Version"

Hope you get it from there, cuz I have never done it and don't know what steps to take after that. Good luck.