Unless it has changed recently - and I don't think it has - MultiMC does not work with the newest modpacks. Due to a change in the way Forge mod works, only modpacks based on Minecraft version 1.12.2 and below work in MultiMC. The new Omnia modpack from FTB is based on Minecraft version 1.15.2
Obviously, this isn't the place to discuss MultiMC since it has nothing to do with FTB, but I am in the same boat, I much prefer to use MultiMC as my multi-purpose Minecraft Launcher. Having said that, I'm very glad SOMEBODY has actually gotten the newest Forge to "play nice" with launchers. Well done, FTB Team!! I am hopeful it is a sign that someday soon, MultiMC will also successfully figure out a way to handle the newest Forge release.