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 Loops 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.