unfortunately all installer make Mountain Lion usb with GUID partition support. You have to enable MBR partition support in Mountain Lion installation usb. Download OSInstall patched files for your Mountain lion (10.8.0 = 10.8 GM) from this link OSInstall(MBR)+OSInstall.mpkg 10.8.x - Downloads - OSx86.net
these two OSInstall files stop the Mountain Lion installer app from checking whether your hard drive has the MBR partition scheme.
Step 1- ShowAllFiles (show all hidden files) (if you don't know how, link Download ShowAllFiles for Mac - Simple app that will show all hidden files. MacUpdate.com download and run, click on show.)
Step 2- open installer drive and go to System/Installation/Packages and delete "OSInstall.mpkg"
Step 3- Copy downloaded OSInstall.mpkg there
Step 4- Now go to System/Library/PrivateFrameworks/Install.framework/Frameworks/OSInstall.framework/Versions/A and delete OSInstall
Step5- Copy downloaded OSInstall there. thats all for MBR
now run installer again and it allows you to install Mountain Lion on MBR partition.
works for me thx.