Hi Adrian,
For changing the frequency, you can take a look at the example "Gen Fig Pulse Train-Buff-Implicit-Cont.vi" in the Example Finder. To access the Example Finder in your VI, go to "Help" and then "Find Examples..." This example shows you how to create a counter task, set up the timing and write a frequency. This example shows you how to write one frequency, but you can modify it to change the frequency during your program operation.
For pausing the counter, you may want to take a look at the example "Gen Dig Pulse Train-Continuous-Pause Trigger.vi" in the Example Finder. This example shows you how to set up pause triggers which allow you to start and stop the counter. Basically, a counter typically has two main inputs: the gate and the source. The gate input is used to toggled the enable and disable property of the counter. Pause triggers work by changing the signal that goes to this gate input.
I hope this helps!
Julianne K
Systems Engineer, Embedded Systems
Certified LabVIEW Architect, Certified LabVIEW Embedded Systems Developer
National Instruments