Error message on macro

I've attached a workbook with a macro that seems to be working as it is supposed to, however, every time I run it, an error message pops up and I don't know what to do to fix it. The error message is -
Run-time error '91':
Object variable or With block variable not set

Anyone know what I need to do to fix?

By the way, I'm not real adept at macros so you will probably be way over my head if you get too technical with your guidance (the awesome Leith Ross wrote the macro seen here).

Thanks!!!