A recent Tumblr change broke the XKit button on the header. It’s now fixed, and button should appear after XKit updates itself.
This is where I would recommend a Update All from the XKit Control Panel, but since it’s inaccessible right now, unfortunately the only solution is to wait for XKit to update itself.
This might take somewhere between a few minutes to an hour, depending on the last time your XKit checked for updates.
Remember when like… a few minutes ago, I mentioned how tumblr broke Xkit?
And now the Xkit-guy has already fixed it.
What a glorious time to be alive.