How to install Optifine to Feed The Beast?

I'm trying to install optifine for minecraft version 1.6.4 for the latest Feed the Beast ModPack, Monster. I experience very low frame rates (1-8 fps most of the time) with this new pack. I know that you used to add the zip file to the pack under the Jar Mod tab, however with the latest launcher (v1.3.5) this tab doesn't exist! So how do I add Optifine? If you put it in with the regular mods the game crashes or it has no affect. Also, any other suggestions for increased frame rate are welcome.