how big is your clam? If it is a little guy less than 3" you can do the following:
An easy way to make the plankton available to them is to place the clam on a flat shell so it can attach which allows an easier time moving the clam. Fill a shallow dish with tank water and add enough plankton to give the water a slight green tint then place the clam that is attached to the shell into the dish for 20-30 minutes so it can feed. If you are just dumping the plankton into the tank it may not get enough, so this is much easier. I don't have any small clams because this is kind of a pain, but that is a good way to do it. There are also many different plankton products on the market do a search to see which one will work best. I think a lot of people use DT's...or you can grow your own!!