Currently, DMDirc has installers for Windows, Linux, and as of DMDirc 0.6, OS X. Users of other platforms will have to manually install DMDirc in order to use it.
First, download the "Java archive" release from the downloads page. This file contains the entire DMDirc application, and all of its associated resources, and can be ran using Java directly without extraction.
To run DMDirc, you just invoke the Java runtime environment with the -jar instruction to tell it the location of the DMDirc jar file. For example, if you are using a unix-like operating system, and the DMDirc jar file is in your home directory, you would use a command like the following:
$ ls DMDirc*.jar DMDirc-0.5.5.jar $ java -jar DMDirc-0.5.5.jar
You may wish to create a shortcut to execute Java in this manner, to save time when launching DMDirc in the future.
If your operating system supports it, you can manually register DMDirc as the protocol handler for irc:// URLs. The process for actually registering the handler is extremely platform specific, but the arguments needed when invoking DMDirc are the same on any platform:
$ java -jar DMDirc-0.5.5.jar -e -c irc://server/channel
The -e switch tells DMDirc to reuse an existing instance if possible, and the -c switch tells it to connect to the specified server and channel.
If you're having problems with DMDirc, please feel free to join us on IRC.