I'm not sure if i am the only one who noticed it, but can anyone else ACTUALLY add a spell to they're spells known from a different class's  at the requisite level? (1st , 8th, 14th, and 20th) The PHB says that at these levels the arcane trickster can take 1 spell from a different class but the character sheet nor the manage level page will let me do it. Am i just an idiot or has their been a general retcon?  

It doesn't say different class, it says different school. 

The spells you learn at 8th, 14th, and 20th level can come from any school of magic.

Arcane Tricksters are normally restricted to Enchantment and Illusion school spells - but the spells at 8th 14th and 20th can come from any school. All of them must come from the Wizard class spell list.

oh excellent, my mistake I had it misread, 

You are not the first person to make that mistake (even in the last week).