In the power pivot screen, I can only access approx 39 of those columns,

What is the process of adding the additional columns. Spefically, the Inventory line PO Original amount
and the inventory line SO original amount. 

I found how you can add more fields to the SQL  table, however I still cannot figure out how to Make assessible the
Inventory line PO original amount and Inventory line SO original amount fields in the power pivot inventory details table.

To add more fields to the SQL table
Power pivot window > select the table (workbook) you want to add more fields > design tab in excel > table properties > scroll right and add check boxes to those fields  you want to add to the table. > click save > in the power pivot window back in excel you will see a REFRESH button,  click to refresh

Went to bed,  came back to it this morning and "voila" it miraculously   the two fields  Original SO and original PO qty appeared in the pivot table field list.  Im sure they were there all the time and I just missed them.