Following this kb article for excel 2007 shows that you need to install the add-in:

to use the Solver Add-in, however, you first need to load it in Excel.

- Click the Microsoft Office Button , and then click Excel Options.
- Click Add-Ins, and then in the Manage box, select Excel Add-ins.
- Click Go.
- In the Add-Ins available box, select the Solver Add-in check box,
and then click OK.
- Tip If Solver Add-in is not listed in the
Add-Ins available box, click Browse to locate the add-in.
- If you get
prompted that the Solver Add-in is not currently installed on your
computer, click Yes to install it.

- After you load the Solver Add-in,
the Solver command is available in the Analysis group on the Data tab.