Pivot Table that performs a distinct count in excel 2013

I'm using 2013, and need to use the count function in the pivot table. However I need it to be a distinct/unique count versus a count of every row. I've read somewhere else that in order to do this, I can hit the check box "add this data to the data model" during the first step of creating a pivot table, and this will enable me to create the distinct count. I see where the checkbox is, however it's grayed out and doesn't even allow for me to check it. Is there a solution to why I can't check the box? Or does someone know if there is another way to do a distinct count in a pivot table? thanks for any assistance.

ok, what I did was copy all the data and paste into a new worksheet and saved it as an xlsx file. I began creating the pivot table, and the box was able to be checked. So thanks for helping out with that step.
But now I'm having troubles with the next step. I add the columns to the pivot table, and go into Value Field Setting for the specific column that I want as Distinct Count. However, there is no choice in the dropdown menu for Distinct Count. The last function to show in the drop down is Varp

I think I solved the last issue.
thanks Izandol for the assistance

I think I solved the last issue.
thanks Izandol for the assistance