I want to put Slaughterhouse Five (by Vonnegut) on my WL, but it is nowhere to be found in the system. I find this really odd, since it's a book that's been around for quite sometime now. How would I go about putting this on my WL if the system won't even pull it up??

Try the same procedure described in the thread for adding pre-release books.  Find the ISBN (from Amazon, for example), then try to add the ISBN to your TBR pile.  Once the book is on your TBR list, then you can go to the book description and add it to your wish list. 

It's 'Slaughter House Five' and it is in the system. Make sure you search All Books and not just the Posted Books.

Wow - What I just found is that it is in the system as both Slaugher House Five and Slaughterhouse-Five (with the hyphen). Amazon lists it in the later way. This could be a bummer for someone who has listed this book is people are searching for it in a different way than it exists on the system. I'm surprised to see it listed two different ways (but not the way that I originally searched for it!!!).

I find sometimes the easiest way to locate a book is by author rather than title.

Vonnegut's books are very popular and go very quickly here. I once posted two 1970s mass market paperbacks of his and they were snapped up in a matter of minutes.

I had no idea!  I see them all the time at Goodwill :-b  I guess I'll start grabbing them and posting them here whenever I see one!