Can you try right-clicking in the upper-left part of the grid?  I think that's how I usually get to the lines fields.

Yes, that did work, thank you!!!

Now I'm trying to figure out what flags need to be set when I import Invoice History lines so that it uses the SO and not the AR format for Invoice History.  I've set the module code to S/O and the order type to "1" on my imported data.  When I create an update an Invoice, it uses the SO form, but my imported data uses the AR form.  I checked all the flags and they seem correct. 

You are importing directly to A/R invoice history?  That's really not a good idea.  There are so many interrelated files that you will create inconsistencies and mismatches in your data.  For example,  you would also need to import to transaction history files,  payment history files,  monthly summaries, etc.

It is best to import to the data entry files and then update.  Temporarily turn off the integration to the general ledger so that it won't post to the general ledger.