You will possibly need to add it to the menus yourself. If you're using KDE, right-click on the 'K' button (or the red hat if you're using redhat) and select Menu Editor. I don't know how to do this in Gnome, sorry, but it's probably not too tricky.
Anyway, to find where it installed to, go into a console as root, and type:
locate openoffice | grep bin
That should give you a list of files containing the word 'openoffice', and also 'bin' (you want the executable files). They should be somewhere like /usr/bin or /usr/local/bin. There should be a few of them, hopefully it'll be easy to tell which file is which. You can now add these to the menu.
If this comes up with no results, try doing a broader search, eg: "locate office | grep bin", "locate openoffice", etc. These should give lots of results, but you should find the files you want. Remember, they will almost certainl be in a "bin" directory
If this doesn't work, I'd probably try:
To list by modification time. Try and find some files matching the time you installed it.
Try the same thing in /usr/local/bin, /bin and you should find the files.