I know this is a pretty old post but this is the only thing on the internet I was able to find showing this exact problem. Posting my solution in case anyone else comes across this looking for answers.
I had to help a user with this exact issue today. Developer tab enabled, Source pane opens but doesn't show the buttons at the bottom. After popping out the Source pane and making it smaller, I could see the buttons. Seems like a display issue.
To fix it, in Excel go to File > Options and go to the General tab, at the top under User Interface options toggle to "Optimize for compatibility" and close and re-open Excel. The buttons should remain visible.
