The free, community version of RapidMiner will only use a single core. You need the enterprise version of RapidMiner to take advantage of multiple cores. You could, however, start a second instance of RapidMiner if you have a separate modeling task to run, and since its a separate process, it could run on one core while the first instance runs on the other.
Keith is right, the support of multicores is available only in the RapidMiner Enterprise Edition (EE). Of course, you could run multiple instances of the community edition as Keith suggested. This will however neither increase the speed of any model building nor any evaluation process may be easily distributed among cores. The Enterprise Edition currently supports multicores in evaluation processes (cross-validation), paramater optimization and decision tree learning. In addition to that, the Enterprise Edition has several other advantages over the community edition including e.g. bugfix guarantees and automatic updates. Please have a look at the following website to obtain more information about the available packages: