(I am assuming that you are talking about EZdrummer 2 here)
The standalone version of EZdrummer 2 (the one that opens up by itself, without loading it in a host, such as Studio One) can't be used inside a host.
In Studio One 3, you need to open EZdrummer 2 just as you open up any other instrument plugin. If you can't get that to work, let us know.
