When the window pops up, you will see a week overview. Click on a field to adjust whether that date&time would be OK for you or not. To select/deselect a whole day or time of the day, click on the weekday or time frame box.
When everybody has made their choices, you can select "Suggest Date" from the "Scheduler" menu to report the best dates to the Chat Commlet included as a plug in in the Scheduler Commlet. You can then discuss anything else there.