Sven Fechner (@simplicitybliss) is one of my major resources when it comes to using OmniFocus. He wrote about linking OmniFocus and Evernote together in at least two blog posts:
In the second post, there is an important comment made by Bryan and Diego. You can link even easier now, by using the Evernote’s ability for copying a note link (or more than one note link).
Getting this note link can be done by selecting one or more Evernote notes and then right-click to get the context menu, and select “Copy Note Link” (or “Copy Note Links” if having selected more than one note):
I prefer to have a hot key assigned to this in the System Preferences. I use the same key for both the menu item “Copy Note Link” (when having one note selected) and the menu item “Copy Note Links” (when having multiple notes selected).
After selecting ⌘K, you can paste the links in you OmniFocus tasks note. Simply clicking on it, will reveal your Evernote note.