A few modifications - your original code didn't initialize a few things. I used number edit box() and lineup box() for a nicer look. Number edit box only allows entry of numbers.
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Water Treatment.jmp");
check_list = {"SS", "SE", "PH"};
todo_list = {};
nw = new window("Checkboxes", << modal(),
text box("Input"),
Lineup Box( N Col( 2 ),
Text Box( "Enter Tail Quantile: " ), tail = number Edit Box() ,
Text Box( "Enter Q Value: " ), quan = number Edit Box()
),
my_cb = checkbox(check_list),
panelbox("Actions",
hlistbox(
button box("OK",
ta = tail << get();
qu = quan << get();
keep_going = 1;
todo_list = my_cb << get selected;
),
button box("Cancel", keep_going = 0)
),
),
);
collist = dt << get column names(string);
filteredcols = {};
For(i = 1, i <= N Items(colList), i++,
If(Contains(colList[i], "SS"), Insert Into(filteredCols, colList[i]))
);
obj = dt << Explore Outliers( Y((Eval(filteredCols) )));
obj << Quantile Range Outliers;
obj << Tail Quantile (ta);
obj << Q(qu);
obj << Select Rows(All);