Please could someone tell me if/where I can add it in, and what to type?
This requires a bit of testing, but at least with the new rasterisers there is an option to enable vsync in GEM+. Click the "Rasterizer" button when you have either of the new rasterisers selected and select the "enable vsync" checkbox. Other than that, your video card driver should have an option to force vsync on. Nvidia control panel has that option which can be configured per each application.