Since you have 10.5, the instructions here may help: http://m10lmac.blogspot.com/2006/11/typing-mongolian-cyrillic.html
I have Macbook air running OSX 10.5, so i can't find mongolian keyboard layout.
I saw mongolian keyboard layout selection availble in OSX Lion 10.7
how do i mongolian keyboard layout comes availble.
There is no mongolian keyboard in 10.7, but you can find them here:
It's not a QWERTY or RUSSIAN layout. It's a true Mongolian keyboard layout (similar to the keyboard layout from Microsoft Windows) It has Cyrillic characters.
I added the pdf in the site and you can access via:
(scroll down to Appendix section in the pdf to see the Normal, Shift, Caps layouts)
Hope this helps.