To add in purlin frames simply follow these few steps.
First select the Truss icon, second select the Tools icon, and third select the Add Purlin Frame icon.
After selecting the Add Purlin Frame a prompt in the lower left corner will ask you to "Select a plane for the Purlin Frame. Press 'Enter' after selection." (I will be selecting the plane inside the red triangle for demonstration.)
After you have selected your plane and hit enter, a prompt will ask you to "Select points for the Purlin Frame. Press 'Enter' after selection."
After you have selected your points and hit enter a dialog box will appear allowing you to enter your requirements for the purlin frame.
After your purlin frame requirements are inserted click OK and your purlin frame will be inserted. You can simply select another plane and repeat the process to add in another purlin frame, or you can escape out of this function if completed.