XYZmoderAqua

When I try to create my own modpack I can't add mod;s that end in .jar but I can do the .zip ones. It says an error message when I put them in the mods folder, and it stays on the technich platform launcher just loading and not oppening another window to go into the modpack. Please help. Do modpacks like buildcraft and thermal expansion have .zip forms instead of .jar.