The most up to date help, with links to other music related topics, is on RecordingBlogs.com.
The loop building tree resides in the left portion of the loop building view and shows the wave files with drum hits that you can use to build drum loops.
All wave files that are shown in the tree reside in the "../samples/" folder of the Orinj installation on your hard drive. The tree mimics the folder structure of "../samples/".
You can listen to the drum hits before you drag them to the track. Click on the drum hit in the tree to select it and then click on the play button under the tree.
To use a drum hit, click on it in the tree and drag it to one of the tracks in the loop building view. If you then add beats to the track and play the loop, you will hear the corresponding drum hit for every beat in the track.
The tree will initially contain the drum hits that come with the Orinj installation. You can include other drum hits (and, in general, other wave files with sounds). To do so, copy these other wave files to the "../samples/" folder of your Orinj installation. You can create any subfolders and organize the samples in any way you want. Once you are done copying, either click on the Reload button beneath the tree or restart Orinj.
See Installation files.