Hi,
I personally still use the 10.0 SDK.
I had a look at the latest SDK because I also use the DisplayExpression object.
In this link, click on Index, then Find (CTRL + F), type DisplayExpression, you will see that the property is within the MobileCacheMapLayerDefinition object.
http://resources.arcgis.com/en/help/windows-mobile-sdk/3.1/apiref/index.html
If you don't mind, once you do get it to work, please share the code for others.
I hope this helps.