There are a number of different reasons for an "unresolved external symbol" error to occur, and your question can be answered better after you provide details of how you call DGEEV in your code, and which library you tell the linker to obtain that routine from.
The simplest guessed explanation is that you have a call to that routine in your Fortran source code, and did not specify that one or more of the Lapack or MKL libraries should be used by the linker.
