Caveat: My answer is based entirely on the SSpower() help file.
The first question is what you wish to obtain the power for. SSpower() is designed to evaluate the statistical power to reject the null hypothesis that given parameter (or set of parameters) is equal to a specific value (e.g., zero) in the population. If that is not what you want, you may have the wrong function. If it is what you want, then you need to decide which parameter or parameters you wish to evaluate the power for.
The key ingredients in any power analysis are the value of the parameter being estimated (a.k.a. effect size), the alpha level, and the sample size. The latter two of these are specified in the SSpower() call using the n and alpha parameters.
The purpose of the population model is to specify the population parameter values. Just as if you were using simulateData(), you should specify the values of all of the parameters in the model. There should be no free parameters because the value of power could vary as a function of any parameter. You are essentially evaluating the counterfactual: How much power would I have for a given (set of) parameters if the world worked a certain way. Your population model is specifying the way the world works according to the antecedent (i.e., condition) of this conditional.
The purpose of the power model (or analysis model) is to specify three things: (a) the model you plan to fit to the data, (b) the parameter(s) for which you want to conduct the power analysis, and (c) the values posited by the null hypothesis. You do the latter two by fixing the parameter(s) of interest to the desired values (e.g., zero for a nil null hypothesis test). Parameters other than those being evaluated and those fixed in the intended analysis can be left free in the power model.
If you are interested in the power of more than one parameter, that is one power calculation for a set of parameters, then specify the number of parameters using the nparam parameter to the SSpower() function.
The result of executing the function call should be a single value representing the statistical power for the above specifications. It may be helpful to compare the above description to the example provide in the help file before attempting your own analysis. The SSpower function can also handle vectors of values, at least for the n parameter.
Keith A. Markus
John Jay College of Criminal Justice, CUNYhttp://jjcweb.jjay.cuny.edu/kmarkus
Frontiers of Test Validity Theory: Measurement, Causation and Meaning.http://www.routledge.com/books/details/9781841692203/