Since I can't see what steps you are actually taking to extrude, I'll take a guess and see if it helps. Make sure that you have the polygon selection as your choice in the properties tray. (the old tools now have new options) Once the polygons you want are selected you should be able to move the mouse with the selection tool over one of the polys chosen and click-n-drag. There are several options regarding how the polys behave as well so either press one of the modifier keys to get or switch to the one you want or manually select it in the properties tray. I really like being able to extrude multiple polygons simultaneously. (Although Amapi has been able to do this for some time, it's new to Carrara.) I don't know if the image helps but there it is anyway...