LogicPro8 manual (p.620):

Logic Pro must index Apple Loops before they are shown in the Loop Browser. Loops can reside in any directory, but you need to tell Logic Pro where to find them.

To add Apple Loops to the Loop Browser:
1 Open a Finder window alongside Logic Pro.
2 Navigate to the folder that contains the Apple Loops.
3 Select the Apple Loops in the folder and drag them into the Loop Browser.

The loops are added to the Apple Loops library and indexed. When this process is finished, the loops will be available directly in the Loop Browser.

If you drag a single loop to the Loop Browser, the loop will be copied to ~/Library/ Audio/Apple Loops/User Loops/SingleFiles.
If you drag a folder of loops that are located on the same drive and partition as the Loop Browser, the loops will be left at their current location, and an alias of the folder will be created in ~/Library/Audio/Apple Loops/User Loops.

If the loops are located on a different drive or partition, you’ll be asked whether you want to copy them to the loop library, or index them at their current location (loops added from optical media will always be copied). If you choose to copy them, the folder containing the loops will be copied to ~/ Library/Audio/Apple Loops/User Loops/. If you choose to index them at their current location, an alias to the folder will be created in ~/Library/Audio/Apple Loops/User Loops/.

Acid Loops in the Loop Browser:
The Loop Browser also displays Acid Loops. Acid Loops don’t contain tags like Apple Loops. In Acid Loops, this information is derived from the surrounding folder structure (especially the names of folders), which must comply with certain standards. This means that it’s not possible to drag an individual Acid Loop file into the Loop Browser. As such, you need to drag the entire CD (or folder) that contains the Acid Loops into the Loop Browser.

If the loops don't show up in the Loop Browswer after being added, the Loop Index files may need to be updated.