If you're using MATLAB 2011b or newer, you likely have access to theCurve Fitting app. If you have this tool installed, you can open it by typingcftool. This tool allows you to import your data and then define a custom equation to fit. Once you've imported your data, you can select "Custon Equation" from the fit type menu and define you're own equation to fit. Thecftool is probably the most straightforward method for fitting nonlinear curves to experimental data. Here is a link to an introduction to the product and an example.