One way for the user to select features in the document is to call the GetFeatures() method of the WorkflowSupport.Document.Map object. GetFeatures() allows the user to select any number of features (either one at a time or through selection windows and crossing windows) until they press the Enter key. The features selected are returned as a FeatureList as follows:
FeatureList features; features = this.WorkflowSupport.Document.Map.GetFeatures();