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.