Is it possible to query from an online PowerBI dataset or dataflow? If it is, could someone explain how to do that?
No, I don't think so.
PowerBI is not on the current list of Data Connectors for JMP.
At the bottom of that page you can make requests for new data connectors for JMP.
Perhaps you can export from PowerBI in a format that JMP can read, such as .csv.
I hope this helps,
Phil