You do not need an IF statement. Better search for the records.
Enter FInd Mode 
Set Field [Days Past Due; ">=45"]
Perform Find 
This will select all records with a payment overdue by 45 or more days.
I may be oversimplifying this, but would this not work?
If [ Days Past Due >= 45 ]
Show Custom Dialog ["Payment"; "Customer has Outstanding Balanace!!!!"]
Thanks for help on this. I believe that is exactly what I needed. :) I'll go test this.