To specify an instance, you should only need to put the instance id in after the attribute name as you have noted. I just created a watch with the following expression:
ipNetToMediaNetAddress.4.172.19.10.10
Threshold example would be “violated if == 172.19.10.10”
And I get an alarm. It sounds like you have an error in your expression. If you are literally putting .X, that doesn’t work, you have to specify the specific instance. If it’s a variable instance that changes, watches won’t work here as watch expressions that have instances defined cannot work for variable instances that change.
Cheers
Jay