That's a little vague. Do you mean you have a method on your plugin class you'd like to call? There's no real way to do that, but you might be able to fake it if you really need to call something on it.
Try something like this:
SPPluginEntryFunc entryFunction = 0;
// check error
void* my_data = 0;
entryFunction("my_caller", "my_selector", my_data);
That won't let you call a method directly, but in your entry function you can catch your custom caller & selector and use that to determine what method to invoke.
All this is the calling side of things.
How do I set up a function as the EntryFunction of my plugin?