To use manual routing, enable it with rtproto, then set each nodes routing tables with the add-route-to-adj-node command. For example:
$ns rtproto Manual set n1 [$ns node] set n2 [$ns node] $ns duplex-link $n1 $n2 10Mb 100ms DropTail $n1 add-route-to-adj-node -default $n2 $n2 add-route-to-adj-node -default $n1
For a more complete example, see tcl/ex/many_tcp.tcl.