It seems to be a reasonable description of Twido as both a Modbus master and slave, their example uses two Twidos's talking to one another, one as a master, the other as a slave.
You'll have to locate Citect Modbus documentation and read through it and then figure which is master/client, which is slave/server and start modifying the example to fit Citect/Twido.
You might consider documenting your step-by-step activities to getting Modbus talking and publishing it here as a Twido/Citect cookbook example.
