Mercury acceleration works only with certain NVidia graphics cards, other than in one or two specific Macs.
So the answer is to buy a new NVidia graphics card - preferably one on the list of approved cards, which Adobe publishes in their information about Premiere Pro.
It is possible to make almost any NVidia card work with Premiere Pro - provided that it has at least 1 GB of memory. There is a simple, well publicised "hack" in which you add the name of your card to a list in a Premiere file in your "C:\Project FilesAdobe\Premiere Pro CS6" folder.
Oops, sorry! I have just realised from your post that you must be referring to a Macbook! In which case forget my second and third paragraphs.
Check the list of Macbooks which do offer Mercury acceleration. If yours is not on the list, I am afraid that at the moment you cannot get the acceleration.
Message was edited by: Alan Craven Realised that original post gave wrong information.
The list is linked inside http://forums.adobe.com/thread/773101
Supported AMD graphics cards for GPU acceleration
- AMD Radeon HD 6750M (only on certain MacBook Pro computers running OS X Lion (10.7.x) with a minimum of 1GB VRAM)
- AMD Radeon HD 6770M (only on certain MacBook Pro computers running OS X Lion (10.7.x) with a minimum of 1GB VRAM)
Your 6750M is listed, but you need to update your Mac software to at least 10.7