I've never installed Joom, but I have done WordPress. Yes, simply create a folder inside of htdocs called something... say MyJoom. Actually, my bet is that if you are supposed to un-zip Joom it will create its own directory. In that case copy the Joom zip file into htdocs folder (XAMPP uses a "shortcut" to it... it is actually in applications... XAMPP... xampfiles but just go to Finder and click the short cut to open the htdocs folder.
When you are using a web-based (Apache) application, stuff always goes in (or below) the htdocs folder. Nothing goes in the Application folder (directory)... except XAMPP of course. The htdocs folder is actually "localhost" under the default set-up of XAMPP. So when you enter "localhost" in the browser it goes to htdocs and looks for the "index.html" or "index.php" file and starts that one. To install Joom you would copy or unzip the Joom package to "MyJoom" and then enter on the browser line:
localhost/MyJoom/install.php (or whatever they tell you to do)
and follow the steps. I assume it will create the MySQL database for you. If not, you will need to do that via the built in phpMyAdmin program that is on the XAMPP home page ... the one that comes up when you enter just: localhost.
Hope this helps.