Saving to the Material library creates either an MT5 file (if you choose "Single material") or an MC6 file (if you choose "Multiple materials").
You find these in a subdirectory of Runtime/Materials of the Runtime that was active when you saved them.
These files only show up in the library palette under Materials (you must be in the Material room to for the Materials category to appear).
Converting an MC6 file to a MAT pose is quite easy with just a text editor (it shouldn't be difficult to find one of the threads that explains how).