You need to make sure you've run tekkit once. Go to
%APPDATA%.technicmodpackstekkitmain
Put "Core" Mods (i.e. .jar files labeled Core) in the Coremods folder in that directory, put mods compatible with forge mod loader in the mods folder. If you want to install Optifine (or something that can't be installed via FML), do it the old fashioned way, but put the files into the Modpack.jar in the Bin folder.