Thank you Jonas (100% work)
First, install Xcode if you do not have it yet. If you installed Xcode
4.3 or later, next open Xcode, go to Preferences -> Downloads and
install the "Command line tools" package (the installer doesn't know
about this limitation because Xcode 4.3 was released after FPC 2.6.0).
Then you will be able to install FPC.
Jonas