try to right click the install program and run as an administrator maybe that help if not try to download it again, you can also right clik on shipsim icon and see where it are installed, normaly it should be installed her on a 64 bit windows C:\Program Files (x86)\Vstep\ShipSimExtremes
and her on a 32 bit windows C:\Program Files\Vstep\ShipSimExtremes
BDW were you bought the D.L.C.
Tore