Why i can't download Mavericks if i have a macbook a1181 2008 with Lion 10.7.5?

http://www.apple.com/support/mac/app-store/contact/

A1181 refers to the generation of white and black thermoplastic MacBooks originally introduced in 2006 and produced through mid 2008.

None of those models meet the minimum system requirements for Mavericks.

https://www.apple.com/osx/specs/

OS X Mavericks: System Requirements

The 2008 A1278 MacBook 5,1 aluminum "Unibody" models are the oldest MacBooks that can install and run OS X 10.9 Mavericks.

, About This Mac, More Info… will help you locate the specific model identifier of the one you have.

Besides searching Apple Support Communities, www.everymac.com is another handy resource with detailed information including specifications of each Mac model.