First off, not being able to ask for instruction from High Witches. I've looked for it at every relationship stage between acquaintance and best friend, it's not under ask, or anywhere else. All the Sims I've tried to turn into witches were Young Adult or older, so that's not the problem.

I turned a few sims into witches/warlocks using treeag's Sim Transformer http://www.modthesims.info/download.php?t=283201, but they can't cast spells on other people, even if they know the spell, have reagents, and are friends or higher with the target sim. There is no option showing up on the menu.

Have you worked through the steps in the FAQ? What happened?

You should get the "Ask to learn" option with sims who are solidly best friends with the witch. A mod conflict might block this - and not necessarily a mod that has anything to do with witchcraft. Work through (don't just read) the steps in the FAQ, and tell us what happened when you did.