The plot command plots only the real parts of the numbers. If you want to plot all your data, you can either plot the absolute magnitude (use the abs function), or plot the real and imaginary parts separately, either in the same plot or using subplot to plot them on different axes in the same figure window.
It may be worth exploring the reason you are getting complex parameters and results. If you can use lsqcurvefit, consider constraining your parameters. (I do not have the Curve Fitting Toolbox, so if you are using it, I cannot help you.)