Slyen,I have an idea for a campigan that takes place during the 5th frontier ware where the PCs are a unit of Mech pilots (yes like in Mech Warrior but more like Mobile Suit Gundam) that are mercs for the Imperium or actual Soildiers of the Imperium. but I dont know how to do it right please any help on designing the Mechs and putting them into a first Adventure and a design for a first Adventure.
Well, depending on which version of Traveller you're using, you may already have rules for designing "walkers" or legged vehicles. (I think you'll find with some experimentation that they won't make military or economic sense, but hey, it's your game, knock yourself out.) I'd play around with those designs and see what the vehicles turn out to be capable of, how big they are, what kind of support requirements they might have, etc. From that point you could determine what kind of ship or ships you need to haul them and their support around.
As for what kind of missions or adventures - at first glance this falls into the classic "mercenary ticket" kind of adventure. Keep in mind that armored units (and these pseudo-Mechs are essentially another type of tank) are great mobile weapons platforms but not especially stealthy. They could be assigned to augment the defense of a particular world against the Zhodani / Vargr / Sword Worlders, or as part of raiding force sent "behind the lines" to attack enemy bases, or as a counter-guerilla force against the Ine Givar.
Of course, the war drags on for a number of years, and your PCs can't spend all their time in their pseudo-Mechs... plenty of time for "dismounted" adventures between combat ops.
John