Minato Mirai 21 is a place easy to get to, here you find the 2 closest stations to this area:
1) The first one is JR Sakuragi-chō Station, on Keihin-Tōhoku and Yokohama Lines.
If you take the JR lines from Tōkyō, take the Tōkaidō (Ueno-Tōkyō) Line first and then change on the Keihin-Tōhoku line at Yokohama Station.
2) The second station is Minato Mirai Station.
Since there is a direct access with Minato Mirai Line, getting to the area from Ikebukuro, Shinjuku and Shibuya is easy.
Minato Mirai Station is located underground, at Landmark Plaza.
You can also check the directions on the article about Yokohama China Town, a few blocks southern to Minato Mirai (so they can be explored by foot as a set). See below:
Yokohama Station isn’t the closest one but it is connected to Minato Mirai 21 through a very pretty elevated walkway where Nissan Motors’ headquarters and exhibition hall are located. Between Nissan Motors showroom and Minatomirai there is still an under-development lot, but the scenic entrance to the skyscraper area is really cool and worth a stroll. Recently (as of 2016), many shops and cafes have opened, making this area also nice. Take a look at these pictures: