You will have to upgrade OSX or return Lightroom for a refund. If you have a Windows PC, you can install it there.
On what basis? Adobe makes the system requirements clear enough.
30 day return period. What's covered and not covered is in the help doc.
My own story: I purchased Photoshop CS5 the first day it came out. I should have trialed it because the Marquee was ghosting on my old Toshiba laptop.
So I applied for that return and it went through.
Only Adobe product I ever returned.