http://www.formula1monaco.com/TicketingSystem.aspx
However, as far as I can far as I am aware there should be dates listed in the black bar under where it says 2018. You should be able to click on these to start the ticket buying process. If you click “facilities” at the top right you’ll get the map with stands and other locations marked but I could see no way from there to initiate any form of ticket buying. You can select your shopping cart but obviously there will be nothing there. If you select the “Aide” option on the top right it shows you how the process should go.
Point that Ed made earlier is that you really need to access via a Windows, using Internet Explorer and having Silverlight installed to get the best options. I reckon this looks a bit overly complex and maybe a simpler solution would be more elegant but it is what it is.