When you go to the optimizer page (the Optimize link on the home page (https://www.askmrrobot.com/, which takes you to https://www.askmrrobot.com/optimizer), you should see three ways to load your character on the screen: AMR ADDON, RECENT CHARACTERS, ARMORY OR LOG. Paste your string into the textbox under the AMR ADDON heading.
If you do not see that, it probably went directly to your last-loaded character. In that case, either click on your character name, or click on the IMPORT FROM ADDON button to the right of your character name at the top of the web page. It will open the same options as above.