Hi there.....
Glad you found the tutorial helpful!
As you've noticed, it's more of an "introduction" than a "complete how-to"... mainly designed to ease people through that initial "set up and have a go" stage... Sooner or later I'll get around to writing an additional chapter or two to cover some of the "next steps", though at the moment and for the currently forseeable future I'm neck-deep in commercial terrain work, so it might be a while.... 
Anyhow..... a fast summary.....
Your primary "objects bank" is of course, the Arma2/OA assets... these were unpacked for you at the initial setup stage by good 'ol Arma2P (hopefully), so you should have a P:\CA\ folder which contains all the standard game objects laid out in the correct folder structure... Don't mess with this structure! - just add the models you want to use into Visitors "Natural" or "Artificial" objects sections and use them on your terrain...
In game, since that P:\CA\ setup mimics the Game's internal structure, all the objects will be "found" and used no problems...
External 3rd party model packs work in exactly the same way.... You need to find out what their internal structure is - and mimic that on your P:\ drive... let's pick a popular one and take a look at a practical example....
Eeeh..... let's look at Berghoff's African Foliage as a popular example of an external models pack...
OK... you've decided you're going to use these trees in your project, you've downloaded the latest Berghoff pack and the result was a "brg_africa_1.23" zip file..... unzip that, and you find the important stuff - "brg_africa.pbo" and it's associated .brg.bisign file.
Since you'll be using these trees on your terrain, they're now a "required addon"... you'll need this pack loaded along with your terrain when it comes to game-time, so stick a copy of both files in your terrain modfolder - that way they'll always load with your terrain when it comes to in-game testing/play...
Of course - when you release your terrain to the public - they'll need a copy of Berghoff's trees too, so you'll either include it in your download, or just tell people it's a requirement... it's a popular pack - they may have it already...
Finally - and you've spotted this bit in Mondkalb's tutorial already - you need to tell your actual terrain itself that it "requires" the trees pack, so - note the actual name of the trees pack... "brg_africa.pbo" - you'll want to add this to the "required addons" section at the beginning of your config.cpp... like this...
Code:
requiredAddons[] = {"Takistan","brg_africa"};
OK..... now - just like Arma2P did automatically with the main game assets - you need to unpack this addon to your P:\ drive... but... where do you put it??
Arma2P knew where to put all the main game content - it assembled it in a rigidly correct order - all in P:\CA\subfolders... how did it know where to put stuff???
Unpack "brg_africa" with whatever pbo unpacking tool you use - such as Mikero's "Eliteness"... you'll get a "brg_africa" folder containing all the models, with a "Data" subfolder containing all the textures... there'll be a config too, but the file we really want to take a look at is called "$PBOPREFIX$...
This is the file that Arma2P reads,,, the main Game reads it too... take a look at it in notepad... it reads "brg_africa" and nothing else...
Now.... imagine a "P:\" tacked on to the beginning of that.....
...this is where you MUST put the unpacked folder on your P:\ drive!
So - place the "brg_africa" folder directly on to your P:\ directory, so you have "P:\brg_africa\<all the tree models>"
Now you can launch Visitor, add the trees to your "Natural Objects" selection - from that location - and use them on your terrain as normal...
When it comes to in-game testing, the terrain will find all the correct assets in the expected places and everything should be fine....
B