If this is one of those inherently obvious things that everyone knows about by now, then I apologize, however I just realized how to do this so I thought I'd share.
How to get a youtube video onto your BB SD card:
Get your youtube link and head to converttube.com
If you want the video and audio, click the .mpg option. If you just want the audio, then click the .mp3.
Paste in your URL and click the "Convert and Download" button.
Save it somewhere to your computer when it's finished.
Put your SD card into the adapter.
Take your SD adapter and flip the switch on the side so that it is unlocked. Place a piece of tape over the switch (being careful not to cover the metal). This is because your SD card is most likely write-protected, and as such your computer flips the switch back to locked once you slide it in.
Copy the mpg/mp3 file you got to the SD card et voila!
I hope someone has found use of this