Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Mūsēum, i, and Mūsīum, ĭi, n., = Μουσεῖον, a seat of the muses, a museum; a place for learned occupations, a library, academy, study, Varr. R. R. 3, 5, 9; Suet. Claud. 42; Spart. Hadr. 20; a grotto, Plin. 36, 21, 42, § 154.