First, have JRuby installed, either from trunk or version 0.9.2 if that version has been released when you read this.
Secondly, gem install ActiveRecord, Camping and all their requirements.
Thirdly, gem install ActiveRecord-JDBC.
Fourth, put you MySQL JDBC driver on your classpath (or any other JDBC driver you care to use).
Download blog.rb from the Camping examples here. Modify it by adding the line
require 'jdbc_adapter'after the other require's at the head of the file.
Finally, you need to create a database file, since JRuby doesn't support the standard choice of camping (which is SQLite3). To do this, create a file that's named (on Linux) $HOME/.campingrc, or on Win32 %APPDATA%/Campingrc. (%APPDATA% is on most Win32-boxes something like c:\Documents and Settings\olagus\Application Data). This file should contain your database definition in standard ActiveRecord fare. My example looks like this:
Of course, you should use your own individual settings and driver information.
Camping does only require that the database exists. It will add migrate the blog application automatically, and as soon as you've started the application by doing:
jruby $JRUBY_HOME/bin/camping blog.rb
it will listen on 3301 and be ready to serve an example blog application.
Have fun and enjoy!