Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
addĭtāmentum, i, n. [addo], an addition, accession, increase: inimicorum, * Cic. Sest. 31, 68: vitae, Sen. Ep. 17, 6: praeter nomen nihil est additamenti, Pseud.Sall. ad Caes. de Rep. Ord. 2: pretii, App. M. 9, 6.