I know I have to use a relay of some sort, but I dont know how to modify an existing program to turn on a relay using a digital output, take some measurements and then turn the relay off again. Any suggestions would be grateful.
You're right you need some reley to switch 30A on and off, and maybe an intermediate relay to drive the main one, since PC cards don't have the power to drive great loads (look at your card AND relay specifications...). Depending on the current ouput (you don't say us if AC or DC) and from your application you could use a SSR... For an application we are now developing with a 6035E, we have used an intermediate driver (ULN2003) to drive a small pcb relay and use his contact to drive a large relay for 150A DC current. (We tend to use intermediate drivers in order to achieve isolation from the field too, not only for power reasons).
In the card's manual there is the connecting circuit for using digital pins as output (once you have configured them in MAX).
In order to use it in LabVIEW you need to use the digital output vi (found in Data Acquisition/Digital I/O palette).
Be careful while writing your software: some relays are very slow in actuation (I saw contactors for hevy DC current loads to take more than 50 msecs in opening and some time less in closing), and you must consider it in order to start using the output current after the contactor is effectively closed.