BPM, rather than %MAX, that is.

I'd like to develop workouts based on a very specific target HR range,
rather than a %MAX , but I can't seem to do this.
I think I see the option, but it's greyed-out so I cannot select it.

Any ideas/suggestions?

For the record, I'm using a Triax Elite HRM (no pod) and running Mac
OSX.

Thanks!