Laura
one way is to go down the .NET route, compile each model to a .net assembly add some appropriate interfaces on the fortran code and your done, see the FTN95 help files and example code that comes with the personal edition.
help files an example of mixed lanuge programming in .NET Contents->.NET PLatform
the harder way is create normal dlls and generate export libraries again see the ftn95 help files there are visual c interoperability examples that come with the compiler.
help files are good go to Contents->WIN32 Platform->Mixed Languge Programming
using the latest version of Visual C++ 2005 express compiler you can even mix the two if you want (though that is not for the faint hearted!)
Really depends how level your C code is and what it is doing and what sorts of data are being passed between the two. do you have some code examples?
Carl