I am not sure exactly what you are trying to do, but I think you must send the Press(1) message to a Fit Model object rather than to a Window object. As in the example below (Run and then click "PRESS"). Works in JMP9 (Mac).
dt = Open( "$English_Sample_Data/Tiretread.jmp" );
fit = Fit Model(
Y( :ABRASION ),
Effects( :SILICA, :SILANE ),
Personality( Standard Least Squares ),
Emphasis( Minimal Report ),
Run(
:ABRASION << {Plot Actual by Predicted( 0 ), Plot Regression( 0 ), Plot Residual by Predicted( 0 ),
Plot Effect Leverage( 0 )}
)
);
dg = Report( fit );
dg << prepend(
Button Box( "PRESS",
fit << press( 1 );
press = dg[Number Col Box( "Press RMSE" )] << Get as Matrix;
press = press[1];
ss = dg[Number Col Box( "Sum of Squares" )] << Get as Matrix;
lastrow = N Row( ss );
sst = ss[lastrow];
predR = 1 - press / sst;
values = dg[Number Col Box( 1 )] << Get as Matrix;
newvalues = J( 1, 6 );
newvalues[1 :: 2] = values[1 :: 2];
newvalues[6] = predR;
newvalues[3 :: 5] = values[3 :: 5];
dg[String Col Box( 1 )] << Add Element( "RSquare Pred" );// Add Values does not work, hence the reordering
dg[Number Col Box( 1 )] << Set Values( newvalues );
)
);