Go to the Simviation Main Site, which has a link in the upper right under the "Simviation Forums". From there, you can click on the links, which will take you to the download pages for that catagory.

Simply click on the link to the download you want.  A download banner/pop-up should appear, click on "download", and when prompted, select "save", then select the download destination.  I created a "FSX Download" folder, and save it there. 

Once downloaded, You un-zip the file, and follow the instructions in the read-me file.  Usually, all you need to do is copy the aircraft folder (which contains just the model, panel, sound and texture folders and an aircraft and an aircraft specific sim file), into the FSX\simobjects\aircraft folder.

It looks like pretty good answer.