Hello, Dave.
Thank you for contacting us.
Look in our Application Support folder by doing this:
• Activate Finder.
• Click in the menu: Go > Go to folder.
• Copy and paste the following path into the small text field which appears:
    ~/Library/Application Support/BookMacster
Examine the window which opens.  The file named "Markster.bmco" is the new, empty bookmarks file which Markster created to recover from the error.  There is probably also a file named "Markster#.bmco".  That was the file that took "too long to open".  Markster renamed it.  If you see such a file with an appropriate modification date, you can try again to open it like this:
• Quit Markster if it is running.
• Delete the file named Markster.bmco
• Rename the file named Markster#.bmco to Markster.bmco.
• Launch Markster.
If the same error occurs, and the file gets renamed to Markster#.bmco again, well, as we say, the "problem is reproducible" and was not just a one-off lightning strike.  If you want to send us that file, I shall have a look and see if data can be recovered.  Or, you can try to restore an older, working Markster.bmco from a Time Machine or whatever other backup you have.
If you want to send us that file, you must zip it first, because it is actually a "package".  to zip it,
• Select the file named "Markster#.bmco"
• Perform a secondary cick on it, by holding down the 'control' key while you click, for example.  A contextual menu will appear.
• From the contextual menu, click the command to "Compress" that file.  A new file whose name ends in ".zip" will appear.
• If it is under 20 MB, 
email us that ".zip" file.
• To clean up, trash the ".zip" file.
Sincerely,
Jerry Krinock