Wireshark-users: [Wireshark-users] How to specify second files for capture comparison


I'm using Win32 version 1.2.1 (SVN 29141).   I'm having trouble using the Compare two capture files feature  (Statistics > Compare).   I may be a dolt, but I don't see how to specify second file.    The help document appears not yet to document this feature.  I've tried searching the wiki, but came up empty.

Can someone clue me in?