Before you generate the output file, you might want to preview it. You can either open the membrane in Jmol, or you can see the textual contents of the pdb-file inside the Pdb File Browser. Both options can be accessed inside the 'File' menu or in the toolbar:
To preview a membrane will first save its temporary pdb-file and then open it in Jmol or the Pdb File Browser
Jmol, while previewing a membrane
Previewing is only possible if a pdb-file of your membrane can be saved currently. That is only the case if it's not empty and not exceeding any limits.
Note that larger membranes may need a couple of seconds to be written and opened afterwards!