The Sega Genesis debugger is arguably Genital's coolest feature. Most of it should be self-explanatory. The m command will modify RAM and registers on the address space. It won't modify ROM, use the p command to do this. Watches are only available when debugging the 68000.
Watches are special conditions which the user is notified of if they become true.
If you are adding a watch, specify the expression as the next argument. The secondary is what is being compared against the primary. It may be a register, memory location, or immediate hexadecimal number. The primary can be any of those things as well. The condition is the condition to test for.