I was having the same issue and after an hour of hacking around in the code I have found what you need to do.
Open application/controllers/InstallerController.php and around line 1108 in the _getDsn function you will find this:
$dsn = "mysql:host={$sDatabaseLocation};port={$sDatabasePort};dbname={$sDatabaseName};"
Change to this
$dsn = "mysql:unix_socket={$sDatabaseLocation};port={$sDatabasePort};dbname={$sDatabaseName};";
And the install will run as normal but in the host box instead of localhost put
/tmp/mysql5.sock