If you still have Adobe Flash on your computer, then log into the game through portal.pixelfederation.com.
Find the button that says "Link". Click it.
On the left of the new window there is a box that says "Generate". Click it.
Then click "Copy".
Now go to your Pixel app and open Settings and click "Link".
Paste into the box that says "This is my new device" and click "Link"
Do the same for Steam.
Now you should have all 3 accounts linked together.
If you have any problems, please write to Support (remembering that they only work office hours Monday to Friday, not at weekends now).