The field probably isn't a checkbox, that isn't a field type I've ever heard of.
It's probably stored as a boolean, but the best way to know is to right click the field and select browse data.
Now you'll know what's stored in there. It might be 0/1, or N/Y, or False/True, etc.
Rather than using the brain dead Select Expert, try using the Report->Edit Selection Formulas->Record and hand coding, you get more control.
Then check the Database->Show SQL Query to see what is being passed to the database.
Please include your software version in future posts.
And selects on LEFT OUTER joined tables are ceretainly allowed by every dataqbase that has a Left Outer, otherwise why would you have it as an option?
Sounds like you might want to read up on databases and SQL in general, it proves very useful when writing reports.
-k