As long as you're not using 'crossval' option, like
rtree = RegressionTree( x, y, 'crossval', 'on' );
The following should work:
[yfit, node] = predict( rtree, test_data );
So, just train a regression tree using "rtree = RegressionTree( train_data, train_label ); and then use it like, [yfit, node] = predict( rtree, test_data )
Young