HELP!! I bought the witcher and downloaded the mac installer threw safari and dragged the download to my desktop and when i open the installer it opens using disk image mounter and says image not recognised. could someone please help

Follow these troubleshooting tips:
http://www.gog.com/support/the_witcher/ibread_first_mac_os_xb_common_troubleshooting_stepsi

Especially tip 4: Copy the game from the DMG file to the Applications folder again - in case it got corrupted....

Check these (except for the tips regarding Boxer) if it still doesn't work:
http://www.gog.com/support/the_witcher/ibread_first_mac_os_xb_faq_and_solutions_for_known_issuesi

Further, the downloaded file may be corrupt, try the GOG Downloader as it makes sure the file is okay.