OFX plugin is installed automatically on Windows system. On macOS please select Options / Install plugins…
If Resolve is open please restart it to reload the OFX plugin.
Using OpenFX plugin in DaVinci Resolve
To easily monitor all the clips in the timeline create an adjustment layer above your clips in the timeline in the Edit Page, and place the OpenFX plugin on top of that layer.
Find Effects in the Toolbox in your Edit Page on the left side of your timeline:
And then drag the OmniScope Connect plugin on top of that Adjustment Clip:
Alternatively, you can use Timeline Node in the Color Page:
Placing an OpenFX plugin in the Timeline node can cause side effects when Caching is enabled in Resolve - the plugin can start sending frames to OmniScope once background renderer is started even if the clip is paused.
To fix that please use the Adjustment Layer approach above.