mail() is part of the PHP core functionality; if it's not there then it means that there was either no mail server when PHP was built, or it was deliberately removed at build time. If you need it, then the admin will have to ensure that the mail server is available (if there is one at all), and rebuild PHP. There are some user notes on the subject on the [man]mail[/man] manual pages.
In short, the admin will need to fix this. If you can't get your admin to do so, you're pretty much out of luck.
Edit: Of course, it may be possible (I'm not able to rule it out) that a quick edit of the [Mail function] section in php.ini is all that's needed...