You have three options here. In all cases you will need to get a flickr api key
Then you need to create a text file called build.properties. type in the line api.key=XXXXXXXXXXXXXXXXXXXXXXXXXXXX where XXXXXXXXXXXXXXXXXXXXXXXXXXXX is your api key.
Then if you haven't already got ant installed or you ide doesn't use ant you will also need to download apache ant from http://ant.apache.org/
Now download the source for flickrmosaic from sourceforge and compile it for yourself either from the command line or via your ide and do some simple stuff for yourself.
Thats option 1
option 2: you can join flickrmosaic as a developer and update the application
option 3: you can use this app as a basis for other apps and, giving credit where it's due, release it as another app.