A local install of Magento may give you a warning to change {{base_url}} for security purposes. I believe this only applies to production environments but it is annoying nonetheless. To solve this, within your Magento admin section, go to System-> Configurations and select the ‘Web’ tab (it’s over on the left under the configuration sidebar). Open the “Unsecure” and “Secure” sections and where you see {{base_url}} (this will be in the Base URL fields of each section… ignore any that say {{unsecure_base_url}} and the like, they don’t matter here) put in your actual base url.

In MAMP you may have something like: http://localhost:8888/magento/ (don’t forget that last slash). This will solve the issue.