Presumably you can find the source files for the loops on your Mac with a bit of searching. You also download new loops from somewhere like http://www.macloops.com/
Once you've located the files you want to add this is how to get them into your songs...
Adding audio sample to GarageBand
Connect the iPad, select it in the left-hand column, open the Apps tab, scroll down to File Sharing, select GarageBand, add files with the button or drag and drop onto the file area. Files should be uncompressed audio in AIFF, WAV, CAF or Apple Loops format at 16bit, 44.1 kHz.
Insert audio sample into song
Open song in GarageBand on iPad, switch to Tracks view, stop if playing and tap |< symbol to reset insert point, tap in the area undeneath existing tracks, click Import... select sample, move/trim as desired.