Hi Geri,
It's an easy target to start slamming what the folks at HD tell DIY'ers because they are so frequently very, very wrong. In this case, OmniGrip should NOT be used under the 18" X 18" porcelain (regardless of what Custom Building Products and Home Depot employees have to say about it). Instead, use Custom Flexbond thinset. This is a high quality, latex modified thinset which will work well in your application.
Also, the tiles should not be set directly over the laminate of the counter. It would be a much better and durable installation if you installed 1/4" cement board (Wonderboard is the type sold at HD) over the laminate, following Custom's directions that are found on their website.
As for grout, I have had problems in the past with Custom grouts, and so have a lot of tile setters I know on internet tile forums. I would instead use (especially because it is a countertop) Laticrete Spectralock grout. It is sold at most Lowe's stores-do you have one near you?
Spectralock is an epoxy-type grout, and this initially scares people away from using it. I assure you that it is quite user-friendly and I will be here to help you with advice if you choose to use it.
As Faithhealer mentioned, let's know more about your porcelain tiles so we can advise whether they should be sealed before grouting, and if you decide to use a cement grout then you will want to seal that, as well.
Two top brands are Miracle sealers and Aqua Mix. You can search on their websites for information about retailers near you. The Custom TIlelab stuff is not good.
I'll check in later when I get home from work.
