Click anywhere on the Tweet, to expand it.
Find the Tweet and click anywhere on the Tweet. This will expand the Tweet and display the date, as well as a "Details" link.
Click the "Details" link.
Directly to the right of the date, is a link that says "Details". Click on this to go directly to the Tweet.
Copy URL from the address bar.
You are now on the page of the specific Tweet. Copy the URL from the top address bar. This is the URL of your Tweet.