I downloaded the extra amps and drum kits when I got the new garageband when Mavericks came out. But I haven't been using them much lately and my MBP is running low on space, so I was hoping I could delete the 10gb worth of addons I downloaded. Does anyone have any idea how I could go about doing this? Thanks.
You'll find the content you're looking for in a couple of different places. Starting from the Macintosh HD (or root level of your boot volume), in Library/Audio/Apple Loops, and Library/Application Support/GarageBand/Instrument Library. Once you've located these folders, you can highlight the folders contained within, and choose 'Get Info' from the File menu to determine the best course of action to free up space. If and when this content is removed, it can always be redownloaded.
You can find this option by choosing Download More Sounds from the GarageBand menu. If you previously purchased this content, select Restore Purchase to install the content.