pxConvertResultsToCSV activity is a final activity, if you want to see it in your search, in the operator preferences, check "Enable diagnostic features" and search again. Or Call this activity from other activity and click on Crosshair icon.
Can you try using Call Rule-Obj-Report-Definition.pxConvertResultsToCSV in your activity.
I am assuming that you have created an activity and using this in one of the steps. If so "call @baseclass.pxConvertResultsToCSV" should work. And the error you mentioned above will be because it is unable to find the rule in the inheritance path but you are mentioning @baseclass explicitly while calling the activity and it should not be the case. Try using Call Rule-Obj-Report-Definition.pxConvertResultsToCSV which is the same activity in Rule-Obj-Report-Definition.