Minecraft-Installer.com
@gaby: In order to switch to another Minecraft version, you have to install the Minecraft version with the Minecraft Launcher first.
Exit the Modinstaller and Minecraft
Start the Minecraft Launcher
Click at lower left on "Edit Profile"
Please choose "Use version" "release X.X.X" (e.g. "release 1.8") in the Profile Editor ("Version Selection")
Save your change by clicking on "Save Profile" (at lower right)
Click on "Play" and WAIT until Minecraft has downloaded all required files and has started completely
Close Minecraft again
You will be able to switch to other Minecraft versions (on condition you have installed them already), if you click top right on the dropdown list on the Installers´ main menu. The Installer only offers Mods for Minecraft versions that are available on your pc and our servers.
15:29:25 on 31.08.2015