There're no buttons.
It's a nasty surprise for many students when they discover that sometimes you actually have to understand the mathematical concepts before you can press buttons on the CAS calculator and get an answer.
.
What? It's not April Fools day where you live ......?
Then I guess I better say that I'm joking and that you go to the MATH A:Trig menu and scroll down to what you want ..... Note that means the same as arccsc .......
p.s. I wasn't joking about the nasty surprise business ......
p.p.s. Read the TI-89 Titanium manual (download it from the TI website if you did what most other students do and lost it 42 seconds after opening the packaging of your calculator).