Hi, do someone know, how to jog the robot via omnicore virtual flexpendant? There is no joystick, and I can not find any instruction for usage.
As you point out there is no joystick on the Omnicore virtual FlexPendant - you will have to use the RobotStudio tools in the home tab for moving (jogging) the robot.
The flexpendant is not really “virtual” as it is the same app that runs on the real pendant 
So there is no simulation of the jogging hardware, at least not currently.
But there is an add-in in RobotApps that you can test:
https://robotapps.robotstudio.com/#/viewApp/0827f98d-e511-4aed-91bc-701836212b2f