Are you going to the little head icon on the upper right side of the screen? What are you trying to use as your siggy?
Also, once the new siggy is saved, all the old posts will show it, so there's no need to do a test post.
Here's what I did.
Click on head icon on upper right side of screen. (accunt options) it pulls up a drop down box, click "preferences".
On left side of screen under avatar click on "signature settings".
From there link the code either directly or in the icon box.
totally diff than how it used to be.