I came across this sentence :
"Vorrei che Marta non dicesse nulla di questo a Mario" ...and it was translated as "I would not like for Marta to say anything about this to Mario"
why is 'dicesse' used instead of present subjunctive 'dica'?
When the verb in the main clause is in the condizionale form, the verb in the subordinative is in the imperfetto form. It's a rule.
Vorrei che tu fossi qui con me. I wish you we here with me.
Mi piacerebbe che tu mi dicessi la verità. I would like you to tell me the truth.
The imperfect subjective is used, after verbs like "vorrei", to express a desire related to a specific hypothetical situation, just like in your example.
If the subjective is used in the subordinate sentence, after the main sentence described a command, then you use the present subjective:
"Voglio (command/imperative) che tu non dica niente di questo a Mario"