When users are invited to set up a subscription, they are sent an email with a unique link allowing them to set up an account.
If the invitation fails to arrive in your inbox, please check your junk mail. Sometimes emails are picked up by spam filters.
Otherwise your email address may have been entered incorrectly into the system. Please check with your administrator.
If you've checked the above, your administrator can re-send the email simply by navigating to the Manage Users screen, locate the user in question and click Resend.