Arlanda is Sweden's largest airport and is located 90 km from Västerås. You can take a bus or taxi directly to Västerås or choose to take the Arlanda Express (train) to Stockholm and then a train from Stockholm to Västerås. 
Tip: You can also rent a car or minibus at Arlanda and then you have full flexibility regarding transport during your time in Sweden.

If you choose to take a ferry from, for example Estonia, you will reach one of Stockholm's ferry locations and from there there will be a bus to Västerås. Bus connection is usually purchased when ordering a ferry trip. Remember to pack with a happy mood and a really good game! :)