With this theorems modules, all the non-AMS modules are using macros from the latex kernel only. For more information see this thread:
https://www.mail-archive.com/[email protected]/msg218016.html
To be able to use the theorems modules, copy the files in Theorems_Layouts2.3 to the layouts folder in LyX's user directory (which can be found in Help->About LyX), and reconfigure lyx via Tools->Reconfigure.
GNU Affero GPL v3.0. See LICENSE.