Launching onOne in Adobe Lightroom can be done two ways and each are unique.
File > Plug-In Extras > ON1
None defined... Learn how to add through the Plug-in Manager here.
Right click > Edit In or Photo > Edit In
(Notice the Layers is not available under the Edit In menu. This is normal because Layers allows multiple images to be sent from Adobe Lightroom and External Editors do not allow this.)
It is important to understand the difference between choosing one individual module versus choosing the Suite. If you select an individual plug-in Effects you can quickly go into Effects and back to Adobe Lightroom, however you are unable to jump between the other modules of ON1.