I can't seem to play on 1.9 or 1.9.4 with the Mekanism mods from the official download page and forge. The game loads vanilla and with forge installed, but as soon as I add the Mekanism files to the /mods folder the launcher is as far as I can get, I do not even get as far as the Mojang splash screen.

It does work with 1.9 though, right? I can't get that to work, either.

Mekanism does not currently have a supported release available for 1.9.4. However, I'm getting close to the final stages of development; I promise you can expect a release soon :)