After installation the app will sync your current work item comments to capture the initial state of the fields. This process might take a while depending on how many work item comments there are.
You can always retrigger this process manually, but there is usually no need to.
The progress of a sync process is shown in the configuration page of the app.
Limitations
Work items with over 1,000 comments cannot be synchronized during the initial sync. As a result, the available work item fields may not reflect the correct state.