did you download the DHR packs and the fixes? if so along with all the other DHR RS there should be a Garrett; it should be called: "24 DHR loco D31" if you still can not find it please present your inquiry to the DHR forum you will be helped (more)
There's nothing wrong with your computer, the main package included a very old (about 1 year off of current) version of the Garratt. The latest version was released on the DHR forum along with the weathered freight cars.http://www.steammachine.com/bdaneal/files/beta/d31mk4.cdp