can't open downloaded appz

i have downloaded a couple of appz which will not open!either with the launch in limewire or dragging the sit file onto the stuffit icon it will do nada! i am finding it most annoying as i spent a day downloading one of them. yes i have tried it with ither downloaded files and it works, and there is more than one file that the problem occurs with so i am presuming it isn't the file.
can anybody help me???