When you first open Loopcloud you will be asked to log in. Enter your e-mail and password. Use the same e-mail and password as your loopmasters.com account.
Loopcloud will then download all the data
about your content (instrument and genre tags, BPM and key, etc). Loopcloud does not download any actual samples, just the tag data for each sample.
You MUST let this download fully complete, or you will have to restart the download when next open Loopcloud. Depending on the size of your library and internet speed, this may take a while, but it only needs to be done once.