Mixcraft supports Acid and Garage Band loops.  Search the Internet for Acid Loops or Apple Loops, download them and then add them into your project by clicking the "Add Sound" button on your Mixcraft toolbar.

Here are some free loops and music beds you can download directly.

Loops are currently accessible in Mixcraft under the Loop Library.  Click the "Library" tab at the bottom of Mixcraft's window. Make sure you have the latest loop library by clicking "Check For Update..." on the "Help" menu.

 
Peace Love Productions Peace Love Productions was one of the first producers to create Acid Loops. You can be certain that the PLP loop standards are the highest because they are tested for quality control by someone who knows. PLP provides a destination to download royalty free music loops, beats, and samples for use in any music software. www.peaceloveproductions.com
SFX Source SFXsource.com provides music loops in a wide array of instruments and bpms for music production, as well as full orchestration loops in many genres for video and film production. Royalty-Free Music Loops
Opuzz Fully mixed music loops (Jazz,R&B,Hip Hop, Dance, Pop)  (These loops do not know what tempo or key they are at and you will have to adjust them manually.) opuzz-free.zip (Music loops and beds.)