Hi Kandex - this may be a little late, but I just ran across this thread.
Nobody spoke to non-Auran sites like magandy.net. What you download from these sites are either .zip files or .cdp files. The .zip files unzip to a collection of .cdp files. Some .zip files are "content files" that contain everything you need to use the asset - route, session, loco, or whatever - together with all their required assets - dependencies.
Individual .cdp files are installed in TRS 2006 by using File|Import CDPs from the Content Manager Plus menu.
If you think you have a content file, you can use File|Import Content.
In either case, after that operation is over, there will be a dialog box that lists the names of all the assets. Click on the button that says something like View in Main Window. Then, in the main window, right click and choose Select All so that everything highlights (blue for me). Then right click again and choose Edit then Commit.
Installation should then be complete.
If you get that uncommitted assets message, I have found that the easy way to get around it is to launch Trainz from the launch button at the top of the CMP. Any uncommitted assets are then committed and you don't even have to identify them. After commit takes place, Trainz launches, and when you are through and close it out, the CMP interface appears again and you have to close it separately.
Back to your question 1 for a minute - yes, most 2004 content will work in 2006. But much of the time, you will be notified that something is wrong with it - by the icons on the CMP that have already been mentioned or by the black screen at the beginning of a Surveyor or Driver session.
Probably more than 90% of the time, in my experience, "faulty" 2004 routes, sessions and even most rolling stock or other things work just fine in 2006. TRS 2004 syntax requires some information that TRS 2006 does not, and so the latter sees this extra information as "faulty". But it does not affect the use of the asset in 2006. So, before worrying over faulty content, try it first.
If there actually is a game-affecting problem with a 2004 asset, refer to Error_Fixing_CMP.pdf, which should be in the stuff that came with TRS 2006.
It might help you to know that installation of assets takes place for our purposes in the TRS2006\Local folder. That is a folder that you will want to keep backed up. TRS2006 may need reinstallation at some point.
When the new Local folder is replaced by the backup, you will have everything at your fingertips again, including any content you may have created.
Dick