How do you get a tank to follow in the blitzkrieg campaign series. The instructions don;'t seem to work.

Tanks, trucks and cars can follow in lead by right-clicking on the unit you wish it to follow, so if you want a massive chain of units i suggest simply following the unit behind to prevent them from stopping one-another as a hoard of units try follow only a specific unit in the front of the chain rather than what is simply a step ahead of them.

Core units are rewarded in the campaign and are carried over between missions and can be swapped for different tanks/armored cars and artillery/AT/AA guns in the Order Upgrades screen before a mission.