HOw big is the anemone? Mysis shrimp are a bit small for an anemone unless it is extremely small.
Feed it Silver Sides or raw shrimp (like the store bought kind). Feed it pieces no larger than its mouth (for you that will be hard unless you remember what the mouth looked like). And put the foot on its tenticles and let the anemone grab the food and pull it towards its mouth - DO NOT force feed it.
If you have proper lighting feeding shouldn't be necessary. The zooantha in the anemone's tissue should provide most if not all the nutrition it needs from the lighting of your aquarium (IF YOU HAVE PROPER LIGHTING).
If you don't have the proper lighting or just need to make yourself feel better by feeding it
feed it about once a week. If it doesn't take the food wait 3 to 6 days and try again. After it isn't taking food for two weeks try another type of food.