When you first login you can see your schedule for the week - scrolling up and down lets you see past and future weeks.
When you click on a scheduled workout you'll see the details of the workout - the description of what you should be doing and sometimes some hints/tips/suggestions in the pre workout notes.
When you have completed the workout just fill in the relevant details - the time/duration etc but more importantly the Post Workout comments. What you enter right in here is automatically emailed to the coach.
You can also upload off different devices such as Garmin or Suunto. When you upload from these the metrics such as duration are automatically filled out.
If you are going to be unavailable - work, travel, family commitment etc then just enter these in. On the day of the event click the [+] side and add a Workout type of 'Other'. Note you can also add an event such as a race and also your Metrics like weight, resting Heart Rate.
When entering unavailable times it can sometimes help to be descriptive ie "Away in Sydney for work but will take running gear"
You can also do all of this (except for uploading) via the iPhone or Android Apps which are available free on their stores.