You can get the value of a column for a given row by subscripting to that row.
For example:
dt=open("$SAMPLE_DATA\Big Class.jmp");
:name[1];
Will return "Katie" which is the name in the first row.
Likewise, you can use a matrix for the subscript.
dt=open("$SAMPLE_DATA\Big Class.jmp");
:name[[2,3,4]];
That returns:
{"LOUISE", "JANE", "JACLYN"}
You can probably see where this is headed. The <<Get Selected Rows message returns a matrix, so you can use that to subscript directly to the column.
dt=open("$SAMPLE_DATA\Big Class.jmp");
rows=dt<<get selected rows;
:name[rows];
-Jeff