Please try to check the Soundpool directory in your file explorer and check with it. Then restart the program. If it is still not recognized, click on the "File Manager" tab next to the "Instruments" one. Find the file's directory from there and add the loops you like.
Please make sure your Music Maker is up-to-date, via the appropriate auto-update option in the "Help" menu. The problem you're experiencing should no longer be a prominent one in the current version. If you do continue to have this problem, there's definitely a way around it, and you will get this to work in the "Loops" manager, without having to resort to the file manager.
Please try this procedure: Open the Music Maker program settings (e.g. via the "P" key), and use the "Rescan Soundpools" button. Once this process is done, you should find the entry for this soundpool in the "Loops" section to be functional, with the sub sections for the various instruments and the individual loops being available.
Alternatively, you can also use the following procedure: Go to the MAGIX content market platform Producer Planet (www.producerplanet.com) and log in with your MAGIX user account. Any soundpools you have obtained through the in-app store in Music Maker will appear in your list of purchased products here, since both platforms share the same back-end system and account. You can now choose to download any missing soundpools as OGG or WAV files (archived into a ZIP file) from there. Please download and unpack the missing pool onto the desired location on your hard drive. Then use the "Add new soundpools" function in the settings menu of the "Loops" tab (under the cog wheel) within Music Maker to index the downloaded files in the database. You might want to unpack the files into the same location to which the shop system would automatically download and extract soundpools. By default, that would be: