Hi fairylover, congratulations on your new D2 tablet and welcome to the forum. Glad you decided to join us here at Android Tablets. The easiest way to transfer the music would be to plug the SD card into the computer and copy them onto it.
The problem is that some downloaded music is copy protected and that may not be visible without rooting the tablet. There is no easy answer to your question but typically downloaded files would go to a download folder. That folder is accessible and you should be able to move them to an SD card. Use the file manager to browse around your internal storage and see if you can find them. If so, then we can tackle moving them.
No, the file manager is the app you use to explore your files and move, copy them. Most tablets come with one already installed. If yours didn't there are several free ones you can download and install.
Astro File Manager and ES File Explorer are both pretty good and there are free versions on the Market.
Since they were free, copy protection (DRM) isn't likely to be an issue and you should be able to copy them without a problem. The music itself is probably a bunch of files ending in .mp3 located in a folder named something like music or songs or the name of the music player on your tablet.