I haven't tried this myself, but the relevant text from the easyjet
website says:
Check-in online
If you opt to check in online you can print out your boarding pass after making your booking (From 60 days before you travel and until two hours prior to the scheduled departure time of your flight). You can then go straight to security when arriving at the airport where airport staff will check your passport and/or other travel documents.
Is this suggesting that you have to do the online check-in as part of the booking transaction, immediately after making the original booking, rather than in the more typical '2-3 days before' window? If so, it would explain the absence of an online checkin function on the home page.
Not 100% clear, I fear.
C.