You can use the Fit Y by X platform. Set measured variable as Y, sampling (1-8, Nominal or Ordinal) as X and Subject as Block. Then under the red triangle pick "Campare Means --> With Control, Dunnett's". You will be asked to specify which level that is treated as control.
Script example:
dt = Open( "$ENGLISH_SAMPLE_DATA/Blood Pressure.jmp" );
dt << Oneway(
Y( :BP 8M ),
X( :Dose ),
Block( :Subject ),
With Control( 1, {"Control"} ),
Comparison Circles( 1 )
);