My VBA expertise is limited so I'll usually start by recording a macro to use the Solver function. After having what I thought was some reasonable looking code, I tried to run the macro but got a "Compile error: Sub or Function not defined."
While in the Visual Basic editor, go to the Help menu and search for the info about Solver. There's a catch to using it, here's what the help says
"Before you use this function, you must establish a reference to the Solver add-in. With a Visual Basic module active, click References on the Tools menu, and then select the Solver.xla check box under Available References. If Solver.xla doesn't appear under Available References, click Browse and open Solver.xla in the \Office\Library subfolder."
After following the above, I had no problem running the macro. Give that a try.
Depending on what your needs are, you could also look at using the Goal Seek function rather than Solver.