I had the same problem in 2010 looking for flights to the Dom Rep for our holiday earlier this year - charter flights do not seem to be included in most of the flight engines.
I think you should make seperate searches direct with the major charter flight companies [Thomson, Monarch, T Cook] although you will get less flexibility and probably a maxium of 3 weeks between outbound and inbound flights.
In the end, we flew with Thomson - no complaints, but only a once a week service to our preferred destination.