This is an old screen shot. You do not need to enable it yourself anymore as every Blogspot blogs are now automatically have OpenID enabled.
Blogspot/Blogger users might be wondering, why they also need to login to their blog account while they never know what is OpenID or enable it.
The truth is, every blog that hosted by Blogspot do have OpenID enabled, be it own domain name that pointed to Blogspot or URLs that end with *.blogger.com.
Your Blogspot blog URL is your OpenID, which means you can also use your Blogspot URL to login to any websites that have OpenID enabled, as long as you logged in to your Blogspot account during the time you are using your web browser.
So every time when you comment in my blog using you Blogspot URL or even login into OpenID enabled website with your Blogspot URL, you will see this screen.
You can just proceed by clicking “Yes, Always”. Either clicking “Yes, Always” or “Yes, Just Once” will enable you to login. But if you click the “Yes, Just Once”, you will be always see this screen again every time to you comment in my blog. If you click “Yes, Always”, you can see my blog’s URL listed under your blog’s “OpenID” tab in your dashboard, which indicated you used your Blogspot blog to login and commented in my blog.