Pages 5.1 underlines some words as misspelled, but does not allow me to "Ignore/Learn Spelling"
I create technical papers, and many words are not recognized by spellchecker. Some words can be learned using control-click but most can't be used at all until added to the dictionary. This is very annoying and time-consuming.

The one things that helps me add words is to use the arrow cursor keys to move the insertion point to the end of the word or past the space and then back a space to "jog" the software into letting you add the word.

If that fails, quitting the app or restarting is sometimes effective.