Open your launcher (I'm assuming you're using the FTB launcher here), select Regrowth, and click the Edit Modpack button. This should bring up two lists of mods: those that you currently have installed, and those that are present in your mods folder, but are currently disabled. Look in the Installed Mods list for MDK, Mekanism, MekanismGenerators, and MekanismTools. I'm guessing that you'll see MDK, Mekanism, and maybe MekanismGenerators, but probably not MekanismTools. Then, download whichever of those modules you're missing from
here, click the Add Mod button in your launcher, find the jar files that you downloaded, and click OK. Now, load up the game, and you should be good to go.
I happen to know that Regrowth changes a lot of recipes for things; just be aware that these changes will not extend to anything you install into the pack. If the difference between the default recipes and the recipes you've become used to in the pack is too great, you can edit the pack's Minetweaker scripts to suit your liking. However, that's beyond the scope of my knowledge.