Tom,
What you're seeing is due to the AB being treated as a "single line" by Oracle. When we execute an AB, Oracle sees it as a single statement so when the error comes back, even though Oracle has listed out where the issue is, "where" the error occurs is on the first line of the AB. When you click on the error, you'll see the first line (Declare) highlighted. If you were to click somewhere else in the AB and click on the error again, it will take you back to the top. Unfortunately, ABs are a weird item and and because they're treated as single statements by Oracle, we have to do the same making it very difficult to debug.
Something that could speed up your debugging also comes from Oracle. If you look in the error message, they tell you where in the AB the error occurs. Use the keyboard shortcut and type in the line number to take you to that line in the code. Hopefully that helps.