I shoulda linked this in a while back (Feb 17th, apparently), when I used it. The bottom line is that you find the package, right-click the appropriate shin-dig (see below), and then use a GUI. Because Oracle said so, that's why.

How can I use Oracle SQL developer to run stored procedures? - Stack Overflow:

I highlight it in the SQL*Developer Object Navigator, invoke the right-click menu and chose Run. (I could use ctrl+F11.) This spawns a pop-up window with a test harness. (Note: If the stored procedure lives in a package, you'll need to right-click the package, not the icon below the package containing the procedure's name; you will then select the sproc from the package's "Target" list when the test harness appears.)