that is odd, well maybe they have it set so you can't see the php.ini file over ftp. either way, do you know how to use SSH? is there a php.ini file in that directory?
most likely, you only need to add the parameters in your php.ini file that you wanted to change. i would guess that the server's php.ini file has all your default parameters, and then anything else you put in your personal php.ini file will override it.
you can use the VI editor to create your php.ini file, however, if you are not familiar with it, you might want to try just uploading one with ftp first.
to use VI, once you are logged in, and in your home directory, type this:
(don't type the $, that is your prompt)
that should open the screen to an editor, type
i to get yourself in insert mode, and then you can start typing.
to save your config, then you hold
ctrl + : and then press
x and <Enter>
It's pretty confusing at first, and I'm not sure why they are having you do this with Putty if you're not that familiar with SSH.
again, i would try to make the file locally in an editor first, then try to upload it via FTP. if that doesn't work, you could try to download
WINSCP, which is an SSH program used to transfer files like you would with an FTP program.
once you did get the file there, it would be interesting if the new values automatically took over. i find it odd again that the host only gave you those instructions. usually, any change in the php.ini configuration has to be followed with restarting the web server.
keep us updated. good luck.