In both cases, the client (assuming it has a non-ITU output) will connect via a transponder or muxponder card. This takes the client signal and converts (with multiplexing in the muxponder case) the client signal into an ITU wavelength.
On the MSTP, the ITU port on the transponder/muxponder is then physically patched into the appropriate port on either ROADM, OADM or Mux/Demux cards.
To directly answer your question, on a hub node equipped with Mux/Demux cards, the Tx from the Transponder is connected to the Mux card, and the Rx connected to the Demux. Likewise, at the node where the wavelength is dropped (eg at an OADM site), there will be an equivalent transpoder connected to the OADM module (Tx and Rx at wavelength xyz).
The transponders/muxponders may be in the same shelf, separate shelves, or you may use a direct ITU connection from devices such as a DWDM GBIC, an SDH DWDM card or 10GE XENPAK from Catalyst 6500.
As long as you have designed the network to take the appropriate interface types into account, you should be fine.
