This is one of those little gems that sort of slips through the cracks. I had a colleague who was extremely frustrated because he could not select the structural model linked into his architectural model!
He had checked worksets, design options, visibility setting, the link type, etc...
Well, with the release of 2014 there are some new buttons on the bottom right hand corner of your Revit window.
"Select Links" is one of them.
What does it do? I will pull the description from Autodesk's Help Page on the Topic:
Select LinksRevit models
Enable the Select Links option when you want to be able to select linked files and individual elements in the links.
Linked files can include the following: To select the entire linked file and all of its elements, move the cursor over the link until it is highlighted, and click.
To select an individual element in the linked file, move the cursor over the element, press TAB to highlight it, and click. If the ability to select links and their elements interferes with your ability to select elements in the project, disable this option.
When Select Links is disabled, you can still snap and align to elements in links.
So next time you can't select your link... You have one more place to look!