By plane
The two nearest towns are Morogoro and Iringa. There are no scheduled flights from Dar to Iringa, Morogoro or Mikumi.
By road
Mikumi National Park is about 250km west of Dar es Salaam and is the closest national park to the city. Driving there takes about 4 hours due to the poor conditions of the road.
Travel agents and tour companies can arrange transportation to Mikumi and through the park. You must ensure the vehicle used is a reliable 4x4, e.g. Land Cruiser and Range Rover. No Rav4 and CRVs. Although it's possible to navigate most of the main roads with a sedan, you will not get the best sightseeing experience.
All such issues should be addressed, clarified and agreed to with the tour company before giving a deposit. Ask for pictures of the vehicles, ensure the have a/c. You may want to inspect the vehicles tires before leaving to ensure they are in good condition.