I highly recommend printing off (on legal size paper) the beach and cliff road pages from the following map and taking it along with you to refer to:
Tensing Pen is a good 13 km south from Boardwalk Village (I assume that's what you're referring to). You will pass by on your way to Tensing Pen.
Mega Mart is a big grocery store. How crowded can depend on the time of day you're there, I assume. I was there at 9 pm on a Saturday night so it wasn't crowded. If you don't want to go there, Value Master or HiLo in Negril will do the trick.
I haven't been to Tensing Pen, but stayed at a non inclusive. One could bring a collapsible cooler....I ended up not having room to pack mine and it was fine without as our cottage was closest to the beach so it was no problem going to get my drinks directly out of my fridge....pretty much like going into the house from my own back yard. There may be people who are either "too cool" or simply couldn't be bothered showing up with groceries.....I don't fall into either of those groups as I don't have much disposable income and finding good value is my way of life. Value is important to me, as well as the convenience of having things in my own room. I figure if my room has a fridge, the resort would expect I'd show up with some groceries.
Have a great time!
Edited: 4:42 pm, September 16, 2012