Mpenza becomes Argyle's highest paid player after signing a one year contract.
"Pressure is everywhere in football, so I'm not really bothered by it. What will make me play well is the team," the 30-year-old told BBC Spotlight.
"If we play as a team I will be able to show what I can do every Saturday."
Mpenza added: "I've been at Manchester City where the pressure was even greater, fighting for a place alongside the likes of Darius Vassell and Georgios Samaras."
Right winger Marin joins on a one-year loan deal from French side Lorient.
The 28-year-old told BBC Spotlight: "I start as a complete unknown to the fans and I have everything to prove at Plymouth which is very exciting.
"I'm a fan of English football and have been following it on French television for many years. I have been waiting for an opportunity to come up to play in England."