You cannot add camera support for newer cameras to an older version of LR by merely downloading profiles.
You have to either update the LR to a version that is newer than your camera or use the DNG Converter that is new enough to create DNGs from the raw files and then those DNGs have enough information for an older version of LR to support them. The issue you’d have is that the DNG Converter probably only runs on the newer OS as well, so you’ll have to find another computer to convert the files with.
You should already have profiles for the 1Ds MK2, but the G12 requires At least Lightroom 3.3. The only way to get profiles for a camera is to download and install a version of Lightroom where support has been added. Camera profiles are not separately available.