I once posted a step by step rundown
here, to make it a bit easier.
I think I'll just copy and hope to be helpful:
Try dek.im/arena. If you have Chrome, the TL;DR version is the following:
1. Open Elements and go to the main menu.
2. In the Chrome menu (which can be found in the upper right corner) go in Tools/Developer Tools (the keyboard shortcut for me is Ctrl+Shift+I)
3. In the pop-up table, click on the 'Network' tab
4. Open Arena. This will make a new item called T503 pop up. Click it. (if you have other tabs given there, click 'Preview', if not, then go to step 5)
5. Copy the full text that is shown in the minitable.
6. Open dek.im/arena in a new tab and paste the code in the corresponding box.
7. Generate deck code and forum code. Profit!