Hi,
I think the first step for you is to read the Modeler guide complete before you start invoking the model. Because what I understand from your comments is that you have not yet generated the service. If yes, and you have not selected the input for the first step as the publishable document it will be throwing an exception while building. Once you build the model it is up to the PRT to see why the model is not triggering. But I recommend you to read the modeler guide first.
If you have already done so please mention the steps you have taken and the problem you are facing.
Ram Challuri
#webMethods#BPM#webMethods-BPMS