In order to verify your site with Bing Webmaster Tools you will first have to sign in to Bing with any Microsoft account.
Once you have signed in you will be taken to the Dashboard for Bing Webmaster Tools. Now click the Add Your Site box in the center of the page.
You will have to enter some information about your site and then you’ll be taken to a page with a meta tag to copy. Only copy what is circled in the screenshot below, you don’t need the entire tag.
Leave this page open, you will be coming back to it to verify your site.
Once you’ve copied that meta tag, go back to your WordPress site and paste it into the Bing Webmaster Verification field on the General Settings screen of All in One SEO Pack, then scroll to the top or bottom of the page and click Update Options.
Now, go back to the Bing Webmaster Tools page that you got the code from and click Verify.
That’s it! You should now be taken to a dashboard where you can access all the Webmaster Tools that Bing offers.