I see that your filter contains expression

the rationale of which is blurred for me
In your Rule Details , press fx > double-click on your formula expression > Filter Data.. > Modify... >
- in the left dropdown choose Column2 (instead The expression)
- in the right dropdown (after is equal to) choose Select a field or group...
and then, having changed to Main data source, select field1 and press OK few times
Run (F5) and it should work
PS (ramblings):
You should put into textbox field2 (linked to data fiels field2 of Main Data Source) the value of Column3 element ( from dropdown field1's linked to secondary data source or embedded XML file), so that its Column2 value is equal to Value: from field1 of Main data source

So, having chosen field1, you create a rule of type action, Run these actions:,
Action:Set a field's value
Field: field2 (textbox in Main Data Source)
Value: press fx > Insert Field or Group... > choose secondary data source sample (Secondary) > choose Column3 > press Filter Data... > Add.. >
on the left dropdown choose Column2
on the right dropdown of Filter Data Dialog (of Insert Formula), press Select a field or Group... and choose field1