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What problem have you got downloading Mountain Lion? Note that you have to upgrade to Snow Leopard if you want to install Mountain Lion so, if you haven't upgraded yet, do it.
Also, if you are running Snow Leopard , open > Software Update to install the latest version. Finally, open App Store and purchase Mountain Lion
If you are running 10.5.8 as stated in your profile, you can't download Mountain Lion.
You need to first purchase Snow Leopard from the Apple Store, install it from the disk, update it to 10.6.8 from Software Update, and then you can purchase and download Mountain Lion from the Mac App Store.
You should also make sure your Mac can run Mountain Lion: http://www.apple.com/osx/specs/
If you don't know which model Macbook Pro you have, see here: http://support.apple.com/kb/HT4132