Actually, you don't need to do a find at all for this - the Go to Related Record  script step will do this for you, if you select the 'Show only related records' option.
To answer your question: when you enter Find mode, fields have no values (except global fields). In addition, when you move to the other table, you may arrive at an unrelated record. So to do it that way, you need to take the parent's id value with you, e.g.:
Set Variable [ $parentID ; Table 1::connection_ID ]
Go to Layout [ Table 2 }
Enter Find Mode 
Set Field [ Table 2::connection_ID ; $parentID ]