The following code segment gets the button's ID, assigns a callback, and manages the button. It assumes that quickHelpDialog was just created.
Widget moreButton;
moreButton = DtHelpQuickDialogGetChild (quickHelpDialog,
DtHELP_QUICK_MORE_BUTTON);
XtAddCallback (moreButton, XmNactivateCallback,
MoreHelpCB, NULL);
XtManageChild (moreButton);
DtHelpDialog(3) man page
DtHelpQuickDialog(3) man page