I can't imagine why Encarta would say that. Merriam-Webster, American Heritage, and the OED fail to mention an alternative plural without the -s. (The same is true for ratatouille. If you want to see for yourself what unusual plurals are indicated, search any of those dictionaries for "octopus" or "deer".)
There is nothing of the Latin that would indicate "elision" can be plural. The Latin word elisio is your run-of-the-mill third declension noun. The plural (nominative and accusative) in Latin is elisiones, which in English is virtually always a simple -s. Cf. L. machinationes with E. "machinations."
My guess is that Encarta is simply wrong or that there might be some specialized, non-standard usage that somehow adopted a singular word for its plural, but I wouldn't trust Encarta to relay that without attestation. You're safer going with the three other dictionaries instead.