Dear Manuel-
To my knowledge only 'real' or derived parameters can be averaged in MARK, not the design matrix coefficients.
The computations to compute model-averaged estimates and unconditional variances are fairly easy to do in a spreadsheet program. An example is provided as part of a homework assignment for my class; it is in the context of known fate models, but the ideas apply:
http://coopunit.forestry.uga.edu/FORS83 ... wn/assign7
Note that there can be some debate about exactly how to do averaging for coefficients that only appear in some models. My example assumes that the parameter exists in all models. See Burnham and Anderson 2nd ed for discussion on this.
Saludos,
Mike