You can install modloadler and another mod that requires modloader at the same time, follow the simple steps below
Download Risugami’s Modloader if you haven’t already. link:
http://www.minecraftforum.net/topic/75440-v125-risugamis-mods-everything-updated/
Open up the Minecraft folder following one of the following 3 steps, depending on your OS (operating system).
Open Bin folder.
Open the minecraft.jar file with 7zip or similar.
Drag the .class files from the Modloader folder into the .jar.
Delete Meta-inf, close the .jar and run Minecraft.exe.
Log in like normal. ModLoader is installed!
To install [most] mods, use the same method. ModLoader is required for some mods.
If you delete anything other than the Meta-inf folder, just delete the Bin folder and log in to Minecraft again. Unfortunately, you have to reinstall the mods again.