Yolo the eye button puts the focus on the preview post which appears at the bottom of the post stream during composing. Even if the button is missing, do you still see the preview in the post stream or not?
How are you hosting your forum? If the button is missing we will need more information about which extensions are installed and at which version(s). You can also try disabling extensions until the problem resolves itself to try to identify which one is responsible.
If you have custom CSS or local extenders, those could also be causing the issue.
EDIT: is the ink pen icon from the WYSIWYG extension? It's a very likely culprit as it changes how the composer and preview work.
