minetweaker has not been updated in a while, and the owner is directing people to a fork called CraftTweaker.
see StanH's comment

If the only reason the Minetweaker API doesn't work for 1.10 is because Minetweaker isn't up to date, this could be a relatively easy fix to give back that very useful tool.

If it's gone because it was a pain to keep working and you don't want to support it anymore, then no worries.

It was actually added back to 1.10 branch last week (the only reason it didn't show up sooner is CraftTweaker did not have a maven repo set up).

Incidentally, I gave up on CraftTweaker about 2 days before finding out about the new maven repo, so all the the MineTweakable things also got exposed in SD's native config file. Now you can use either method.

@jaquadro You are awesome! thanks!