Lyanne -
It sounds like you are interested in model-averaged beta estimates, which I don't think can be obtained in MARK (although MARK will provide you with model-averaged parameters (e.g. phi), which is a considerably more complex task).
Nevertheless, it is possible to obtain model averaged beta estimates (and a model-averaged intercept) if you are willing to export AIC's, parameter estimates and SE's to a spreadsheet. I have posted a model averaging spreadsheet on my web site that may facilitate this task:
http://www.uvm.edu/~bmitchel/software.html (bottom of the page).
Brian