Multilink frame relay is basically a bunch of messages that go between the frame switch and the router. These are attached to the router exactly the same as any other frame circuit is done, you don't need anything special.
Although there are a number of options you in most cases just put encapsulation frame-relay MFRx on you physical interfaces and define the MFRx interface.
The only issue is that your service provider has to offer this service. You cannot just bundle normal frame circuits together.
