Check the content you're using is compatible with your version of trainz. Si3D Only supports trainz 2009 fully, we cannot always help with later versions which sometimes break content, since we don't own the software. Content from the classics site is for trainz 2006 only and is no longer supported.
Make sure you have right clicked the asset in content manager and clicked "download", this will commonly download what it needs straight away.
Go to the help section of our forums and find the KUID catalogue, this lists all common kuid requests.
Enter the Kuid into google, it might sound like a long shot but surprisingly it quite often provides results.
If it is for a route make sure you have followed the links that it says to on our routes page and downloaded all content from there.
If you've used this service before, check the e-mail replies for helpful feedback from our team on how to find kuids from similar sources.
Done the above and still no luck? No problem, use the form below and we'll try to help. Please remember we cannot e-mail you any assets we don't own, we can only tell you where to find them. That isn't us being difficult or unhelpful, it's the trainz rules which we pride ourselves in following.
To find the missing kuid right click the asset in content manager and select "view dependencies" This will show you a list, it is the unknown ones with a grey box and question mark next to them that you need to report here. Please don't report buildings you think are missing from routes when you look at it, because chances are the asset is included it just needs putting on the route.