To renew your membership, Log In at the upper right, using the email address at which you receive email from PS SIGCHI. (If you've forgotten your password, click the Forgot Password link.)
Once logged in, click the View Profile link at the upper right. On your Profile page, you can click the "Renew until [date]" button to renew your membership. (If you have enabled automatic renewals through your PayPal account, you will see a message to this effect instead of the Renew button.)
If you do not see a "Renew until [date]" button on your Profile page, try clicking the "Invoices and payments" link at the top of your Profile page to access and pay your already-billed renewal.