Are you on a shared hosting plan or do you manage your own Vps/Dedicated box?
If the former, then there is nothing that you can do about it. Some hostsmay choose to disable the PHP function phpinfo() for security reasons, because it displays information which may be used to compromise the server that your site is running on. In that case you should contact your host and ask them if they can enable it.
If the later, at the php.ini file find the following line: (it's located athe disable_functions directive)
disable_functions = phpinfo
and change it to disable_functions = and save the changes. That should enable phpinfo. Hope it helps.