It sounds like you ware running up against the situation where you are telling the new "one-click" to install to a directory that already contains files (your webcalendar application).
Dreamhost "one-clicks" require that the destination directory be completely empty before they will install the new application. The way you get around this is you specify a new directory/subdirectory to hold the "new" install.
For instance, Let's say you installed webcalendar to "yourdomain.tld", which puts the webcalender application application in the directory "/home/.dataglob/yourdomain.tld/" (using the default method of Dreamhost using yourdomain.tld as the web directory of a domain). Further attempts to install another "one-click" into that directory will fail for the reason previously explained. You could work around this by telling the "one-click" installer to install, say WordPress into the directory "/home/.dataglob/yourdomian.tld/blog" (which would be reachable via "http://yourdomain.tld/blog), and it would work just fine.
Additionally, you could create subdomains for each of your applications. I sometimes do this, as it cost nothing extra at Dreamhost and is a very good way to organized things. You could create domains likr "blog.yourdomian.tld", "gallery.yourdomian.tld", "webcalendar.yourdomian.tld" and just install the appropriate "one-click" program into the appropriate domain. One advantage of doing it this way is that you don't have to worry about any potential conflicts that could arise as a result of having various .htaccess file (often containing Apache re-write rules) conflicting with on another).