Just keeping break point in activated user exit function module will not go to debug mode in case of Outbound IDoc.
There are some specific steps to be followed to debug an Outbound IDoc. (Just Google it).
For easy reference steps to be followed are as below.
1. Set the Break point at the coding part required (FM or Exit).
2. Go to the document (SO/PO etc..) Output messages. Set Dispatch time as 'Send with Periodically Scheduled Job'.
3. Then use transaction WE15 to go to debug mode.