Try this:
String[] options = new String[2];
options[0] = "-R"; // "range"
options[1] = "1"; // first attribute
weka.filters.unsupervised.attribute.StringToNominal ff=new weka.filters.unsupervised.attribute.StringToNominal(); // new instance of filter
ff.setOptions(options); // set options
ff.setInputFormat(data1); // inform filter about dataset **AFTER** setting options
Instances data2 = Filter.useFilter(data1, ff);
and then:
apriori.buildAssociations(data2);