If you can't download sounds from the library, here are some things to try:
Make sure that you are connected to the Internet. Try launching your Internet browser and going to a news article or website that you haven't been to. (This verifies that you aren't viewing cached content.)
Make sure that your security software or Internet firewall is not blocking Mixcraft from accessing the Internet. You will need to go into your firewall or security software and allow Mixcraft access to the Internet. (You may need to use your security or fire wall's help manual or documentation to figure this out.)
Try a different Internet download server by changing Mixcraft's Loop Library Download option in the General preferences (File Menu > Preferences > Library).
If you haven't purchased the software yet, you can always buy the boxed version with all the loops on the Mixcraft buy page.
Download a full installer (without the loops) here: Download Mixcraft 6 Installer (For Non-Internet Computers)
Older versions of Mixcraft:
Download a "full" version of Mixcraft 5 without the loops here: full version of Mixcraft 5. This will allow you to install the software without an Internet connection (you can download it on a computer that has an Internet connection, put it on a CD or a USB drive and install it from there), but it does not include the loops.
Download a "full" version of Mixcraft 4 without the loops here: full version of Mixcraft 4.
You can download the latest version of Mixcraft 4 with ALL loops embedded here (480 MB). (Please note that this is a .zip file and is 480 MB, which could take hours depending on your Internet connection!) You will need to unzip it and then run the install
Mixcraft 3 users can download a copy of Mixcraft 3 with all the sounds here.