[confused] I'm assuming that you have made a 'typo'
shizzle
... a TIFF file is something entirely different from a TTF file!
IF, in fact, you've downloaded a TTF (True Type Font) and have installed it as described you must first shut down Photoshop and restart it. This gives Photoshop the opportunity to recognize the newly installed font!
Now, if you can't find it when you go to your font drop-down list in Photoshop, I will caution you that "sometimes" (make that a lot of times) the font name will be sorted by Photoshop in such a way that doesn't make sense to you. It may have a hidden file name (different from the name that you downloaded) and PS will sort it based on that hidden (true) name.
This is where having a good
font manager makes sense. You can launch the application and find the new font and see its real name. Now it's just a matter of scrolling through Photoshop's font list...
I hope that this helps!
... and... a BIG WELCOME to the board!