When viewing an 1850 census record, I wanted to attach the document to a family member, but the box in the upper right-hand corner was grayed out. What can I do to make that box available?
The "Attach to Family Tree" button on a record image is grayed out if there are index entries associated with the image. This is because FamilySearch considers the index to be primary, and the image to be secondary. Yes, this is exactly backwards from how things should be, but I don't expect it to ever change, because indexes are machine-parseable, while images are not.
What you can do is open the "Image Index" tab at the bottom of the image, find the entry you need, and click the piece-of-paper icon to the left of the name. This takes you to the index page for that entry, where you'll find an "Attach to Family Tree" button at the right. This opens Source Linker.
Alternately, you can stick with attaching the image, ignoring the index, by using the "Source Box" button (next to the grayed-out "Attach to Family Tree" button). Note that it opens an immovable popup window right over the top of the image window, so you may want to copy-and-paste the image URL into another tab first, so that you can actually look at the image while creating the source citation for it. Once you have created the citation, you will need to go to the Sources tab of the profile you want to attach it to, and use "Add Source - Attach From Source Box".
If you feel this page might be of personal use at some later stage, you should also download it to your hard drive / memory stick. There is no guarantee it will always be available to view via FamilySearch.