Currently (as far as I can see) the only way to automate parameters in plugins is to manually select the parameter within *instr Automation. This works well for some plugins, but for others I've found it to be extreamly painful. For example, many synths in Reaktor have a huge amount of parameters and are often not named and instead given a number. The synth I was using just now actually had 1000 parameters, all just given a number.
Now, seeing a list of numbers from 0 to 1000 and trying to work out which is the parameter you want to automate obviously isn't fun. I've found a work-around by looking at the properties and seeing what the ID is, but it could be a lot better.
In other DAWs such as Ableton, you can simply click on the knob for the parameter you want to automate, and then add it as an automation track, this is the feature I would like to see in Renoise. It would clear this whole mess up and make automating plugins a hell of a lot easier.
I've been really impressed with Renoise, and I wouldn't be surprised if this was already a feature that I just missed, so if it is please let me know if it is!