This is just a guess, but I seem to remember that many of these marine HF rigs have two modes... a channelized mode for marine channels and a "VFO" mode for non-channelized or amateur operation. Have you checked in the M710 manual?
Use VFO mode and say 50 Khz step. the rotary encoder or up/down will go through frequencies.
Us the keypad for direct frequency entry and you can even save that to a user channel.
In channel mode, you need to program it for simplex or ad a related TX frequency to transmit.
I forgot one thing, there are 3 flavors of this radio, the "general" wont program for TX.
All you get are what is ITU canned channels, but decent general coverage receiver.
This is just a guess, but I seem to remember that many of these marine HF rigs have two modes... a channelized mode for marine channels and a "VFO" mode for non-channelized or amateur operation. Have you checked in the M710 manual?