I am playing pkmn diamond and after beating the 4 gum leader I don't know..

Try to get the 5th badge?? You want to go back to Hearthome City and battle Fantina [ghost-user gym leader]. With her badge, you CAN use surf, and you then have access to go to the next destination, Canalave City, to get the 6th badge.

Go back to Hearthome City. The Fifth Gym Leader is located there.

Munchlax is gained by breeding a Snorlax, or using Honey on the Sweet Honey Tree near Eterna Forest.

Lucario is gained by evolving Riolu, you get Riolu from an Egg gained on Iron Mountain.

Ok I can't help you with the first part but I know how to get munchlax and lucario. munchlax can be found in many different places. honey trees:routes 205(floaroma town side), 206,207,208,209,210,210(mist),211(celestic town side)212,212(rain),213,214,215,218,221,222, Floaroma meadow, valley window works, and eterna forest(note:you must put honey on the trees so that means there is no guarantee you will get him. now to get lucario you go to a place called iron island you will receive an egg when it hatches you will get a Pokemon called Riolu it is the pre-evolved form of Lucario. it evolves through a friendship evolution so always keep riolu in your party(works faster if it is in the first slot) it only evolves between 4:00a.m. And 8:00p.m. hope it helps!

Ok you have 2 go to pastoria city and theres a guy from team galatic and he says something about he can't wait 2 get a package if you helped out Dawn to get her pokedex from team galatic,the team galatic guy will get the pacage and you will have to follow him to lake valor but before that you will have to battle your partner and when you follow team galatic to the lake a lady named cynthia will give you Secretpotion to give to those psyducks(make sure you have defog it's in the great marsh talk to the lady in the the great marsh game she will say that she found a Hm but she doesnt want it!)and when you get those psyducks out the way the cynthia will give you something to go to give her grandma in Celestic town when you do that you have to go inside a temple or wutever and theres carvings of three rare Pokemon and when you click on them the old lady will give you surf and then when you get that go to Heartthrome city to and she is in the gym.