How to add external native dependency dll?

You may try this:

1. Add/Existing Item, instead of Add Reference.
2. Use Add As Link.
3. Make sure the item is to be copied in build folder. In the property of the library in VS, set Build Action to Content and Copy to Output Directory to Copy if Newer.
4. Done. Rebuild and test.

Suggested in the link http://social.msdn.microsoft.com/Forums/en-US/1b1b316a-8648-4243-a651-84de51fd2508/reference-native-dll-from-managed-c-project?forum=vssmartdevicesvbcs.