Actually, you need Morphs++ only for the full body morphs (Amazon and so on) which are pseudo morphs made up of sub-morphs from the Muscle and ++ sets.

The Muscle Morphs appear as a separate group, under the other morph groups for Victoria 4.1 (not Victoria 4 plain). If they aren't appearing make sure that you aren't using the Rem poses, that you installed to the same folder as for the base figure (and Morphs++ if you have it) and that you ran the updater utility (or for Windows allowed the installer to run it),