What kind of purple tip do you have, there's a few with purple tips.
How long have you had it?
Which tank is it in?
Is it attached?
What condition is it in?
Sometimes if it's new, it may not eat right away. Most people here recommend waiting until a nem is attached before feeding it. Another reason they might not eat is because they're stressed, this can be from unstable tank conditions, adding it to a new tank, improper lighting, etc...
Some nems are picky eaters and wait for that one special food to come along before they eat, others aren't quite as picky. It varies even within a species. Sometimes you have to try a little bit of everything before you find the thing it likes. Ours seems partial to scallops, but will eat silversides very easily too. You can try shrimp, clam, scallop, mussell, meaty things like that from the seafood counter at your grocery store. There's also meaty foods at your LFS like silversides. I've heard people feed krill too, but I'm not familiar enough with it to know where to find it. Usually just putting the food near the mouth is good enough. Ours loves eating and is settled in enough that we just make sure it gets on the disk, he gets it to his mouth just fine now.
Another really good resource for anemone information is the FAQ sticky at the top of the forum. Great info on a variety of nems.
Good luck!