Mac OS X 10.7.5 doesn't work with logic pro 9.1.1

I have a Mac book pro OS X 10.7.5.

I also have Logic pro 9.1.1

My brother used to use the same mac software and logic pro software and it worked.

However when I download logic pro and open Logic Pro a message pops up saying 'You can't use this version of the application Logic Pro with this version of Mac OS X' and then says in smaller writing 'You have Logic Pro 9.1.1'.

But when I look in the 'before you install logic pro' document it says the minimum requirement is a Mac OS X 10.5.7.

Shouldn't I be able to load it? Or do I need to update the mac? My mac also tells me there is a logic pro update available. Shall I update Logic Pro?

Please help me! Thanks!