You can find the Weka code in a file called "weka-src.jar", which should be located in the directory where Weka was installed. This "weka-src.jar" file is a zip file. Hence you need to winzip or unzip it to extract its contents. Inside, you will find the .java files that implement Weka.
Read in detail the "Explorer Guide" and the "Experimenter Tutorial" provided with the Weka system. Browse through the "Package Documentation" to become familiar with it.
Convert the census-income data to the arff format. For this you can either use any tools provided by Weka, or you can make the conversion outside the Weka system using other tools (e.g., a word editor, Excel, etc.). Create a census-income.arff file with the converted dataset.