Well, first off... If you ever have trouble, feel free to ask for help on the forums
You don't need to be a computer wiz to be able to use, and get a lot out of, Trainz

Sometimes a creator will make it difficult (not their intention, by any means), and sometimes things can go wrong. But thats going to happen with any game.
For the most part, the info you need to be able to install is in your manual
But, if this has gone missing, or your version didn't come with one, here is a run down for installing content off the
Download Station :
TRS04: click the download (helper) button. Should automatically bring up the download helper window and start downloading off the DLS. Will install automatically, and should bring all 'dependencies' down with it. If something doesn't work, then use
Trainz Objectz. As above, feel free to ask for help. We should be able to answer any Q's you have.
TRS06/TC/TS09: Click the download (helper) button. This will open content manager plus. Once you have found all of the items you want to download, click the 'start' button in the download window of content manager plus. It will automatically install the items when it downloads them. CMP/CM2 will tell you if any dependencies are missing (if they aren't on the DLS, then normally you should just forget that item, as they can be a pain to find, and again not auran's fault).
For items you download elsewhere, it goes as follows (so long as they are in .cdp format, or a TRS04-TS09 compatible .exe file):
TRS04: Double click the .cdp file, click yes, then click install. Now run Trainz, and see if the item shows. If not, run Trainz Objectz, look for the item (by name) in the list. Right click, and check the dependencies. If any are listed as missing, search the DLS for them (or the site you downloaded from). If they don't turn up, then forget the item.
TRS06/TC/TS09: Double click the .cdp file. This will open CMP/CM2. Go to the 'open for edit tab'. Then select all of the items listed, and hit 'ctrl'+'m' to 'commit' the items. If they show any missing dependencies, then search the DLS for them (or the site you downloaded from). If they don't turn up, then forget the item.
Hope this helps.
Zec