Up until the more recent version of MusicMaker, any instrument that came with MusicMaker was available in Samplitude. Unfortunately, that no longer works. Any new instruments obtained through the Store, even due to using the coupons for Plus and Premium, that were not already installed and available for Samplitude, will not work in Samplitude. This has been raised with Magix more than once and has not been resolved. Hopefully they will allow the use of these in the next version of Samplitude Pro X.
The same applies to Soundpools.
You may want to raise a ticket with Magix to remind them.