As said,a dimmer starts a dimming cycle when it receives two or more on commands, with this function it's possible to adjust the dimming level from a wall switch/remote control. This is described in the different Nexa manuals. There they also state that motions sensors etc are not suitable for dimmers.
Turning on/off a dimmer from a wall switch should work OK.
You don't give much details about your system, but I assume that you have some Tellstick system as your platform.
Then you need to remove the pairing between sensor and dimmer, then:
- add the dimmer as a device in Live, verify that you can turn on the dimmer/lamp and adjust the dimming level
- add the sensor as a device in Live, verify that the lamp icon goes on/off when the sensor is triggered
- create an event, triggered by a on-signal from the sensor, and as an action you turn on the dimmer, with a selected dimming level
- create an event, triggered by an off-signal from the sensor, s an action you turn off the dimmer.
When using an event, there will be a small delay between sensor activation and activation of the receiver.