The code you just pasted is different than the code I pasted above. Copy the code I pasted earlier exactly as it is inside your page_portfolio.php file and it should work.
I tested your code and noticed the apostrophes were different, and the last line of code didn’t match as well. So the issue appears to be just the code itself.
Here is what happens for me once I select the “Excerpts” option in the Portfolio settings and add the code I pasted earlier – http://i.imgur.com/nLIAh.png
My last statement was actually referring to adding more tags in your posts if you haven’t already, but that shouldn’t effect rather the read more button shows or not. Again, the reason it wasn’t showing was due to your code being different than what I pasted earlier. So once you add in the correct code above, it should work, and you can rename the “Read More” to whatever you want it to say….just make sure and leave everything else as is or it will cause another error.
Let me know if you get it working.