This is worth a blog post since it turned out to be a task that took me 15 minutes to complete...
I wanted to sign up for Google Checkout as a seller and tried to sign in with my Google Apps account. The error message I got told me that my account was not enabled for Google Checkout and that the administrator for my Google Apps domain had to enable it.
Luckily that administrator is me so I went on the hunt for the option that enables Google Checkout for my Google Apps account... 15 minutes later I found it, and to save you some time here it is:
1) Go to your Google Apps account at https://www.google.com/a and log in as the domain admin. You may find it easiest to simply log into your Google Apps GMail account instead in which case you now need to click on 'Manage this domain' at the top right.
2) You're now on the Google Apps homepage for this domain. Click on 'Organization & users' (second option, top left)

3) Next click on 'Services' (next to the selected 'Users' tab (it's easy to miss).
4) Scroll down to 'Other Google Services' and past it until you find Google Checkout. Turn it on.
You are done!

#1 by GoGas on 11/7/11 - 2:27 PM
#2 by Stefan Richter on 11/7/11 - 4:28 PM
#3 by Mr S on 12/17/11 - 10:15 PM
#4 by Stefan Richter on 12/19/11 - 2:08 PM
#5 by michele on 10/9/12 - 10:14 AM
#6 by Stefan Richter on 10/9/12 - 11:05 AM
are you serious? Did Google really send you here? If so that's very cool :-)
#7 by michele on 10/9/12 - 11:31 AM
#8 by Stefan Richter on 10/9/12 - 12:42 PM
yeah it's called Wallet now here too, I believe google renamed it and merged the two services.
Hmm, but it does turn it on as a seller for me, for that domain. Are you definitely logged in as the domain owner? If you are then I'm sorry, the steps above are all I had to do to get it to work... :-/
#9 by Olivier on 10/15/12 - 4:36 PM
#10 by softineer on 12/2/12 - 7:42 AM
for sellers, go to this URL:
http://checkout.google.com/sell/