After you log in, do you see a bar at the top of the screen that says, "Welcome back! As a SILVER Castaway Club Member, you are eligible to book new itineraries for summer 2020 cruises.
Learn More"? If you see that, then click on "learn more," which takes you to a page describing the itineraries, with a blue box on the right side that has a button that says "Explore Itineraries Now." Click on that, and then it should show you the summer options. They didn't appear for me when I used the drop-down box on the homepage, but did when I clicked through via that banner.
We were on hold for 3 hours today with Costco, BTW; gave them our info and they said they would call back when they booked the reservation for us, but that hasn't happened yet. And in the meantime, that cruise that included Dubrovnik appears to be completely sold out!