By bus
From Iran: One comes to Pakistan from Iran via the Mirjaveh border in Iran which is half an hours drive from Zahedan. The Pakistani border town is called Taftan and has facilities of immigration, customs, hotels etc, although it's a much better idea to head directly to Quetta and not stick around. There are buses to Quetta in the early afternoon, taking 12-14 hours.
By plane
Karachi, Lahore, and Islamabad are the main gateways to Pakistan by air. However, there are 134 airfields in Pakistan. Six other international airports are in Peshawar, Quetta, Faisalabad, Turbat, Multan and Gwadar.
By train
Balochistan has train links with other parts of Pakistan and Iran, though none of these trains are the fastest or most practical way to enter Pakistan. Should speed be a priority it is better to take the bus, or if you are really in a hurry, to fly, however the trains are sights in their own right.
Iran has one link, from Zahedan to Quetta.