If you just highlight and right click, then copy, you will have it ready for the next step. (By the way... if you are wanting to link back to an old post of your's, be sure to just right click on the title of the post, and it will take you exactly where you are wanting to go, but if it is a picture you are wanting to link to.. just right click on that picture).
When you click on this button, you will see this box:
In the space where it says URL: just past the link that you already copied, and then click, "OK". It should turn the words a different color, and then that should have created your link.
Again... I hope this made sense... this is my first attempt at "teaching", and I am not very objective of my own directions. Good luck! :)