Why is it when I right-click on a folder in Tortoise SVN, click on Properties and then Click 'New...' in the property name dropdown, the svn:externals is missing?

I'm new to subversion/tortoise. However I want to evaluate the software. I wanted to try out the externals feature but I'm stumped at the first hurdle.

If I enter svn:external manually and then set the folder to point to the 'trunk' folder, I get the following error:
Error parsing svn:externals property on 'file:///...' etc.

Where am I going wrong?