J1.5 installation unsuccessful

I installed the J4Schema free plugin into Joomla 1.5.11 and get the message:

Uh oh! There's something wrong
Check the requirements below
Mootools System upgrade not enabled.
You MUST enable it in order to use J4Schema

However, there doesn't seem to be a Mootools for Joomla 1.5 or am I just too inexperienced with Joomla setups?

Please help if any of you know what to do to enable "Mootools System update"