You need to select the field from the list of fields that appears in the dialog. There's a popup menu just above the list for selecting the table (or rather table occurrence) from which the fields are displayed.
Yes, I understand how to enter the table occurence and field under "Find records when"on the left but on the right in the "Criteria" I would like to specify a global field from another table. I want the "Criteria:"to be based on a record from a different table.
For example on the left, Find Records When ledger::date = on the right (under Criteria) Global::dat
That is not possible: stored find requests are literal (static). They cannot contain a reference to a (dynamic) data source.
You need to swich to a different method:
Enter Find Mode 
Set Field [ ledger::date ; Global::dat ]
Perform Find