the login for 'admin', in WordPress, probally is NOT the user id/password that you need to connect to your host. The 'admin' user id/password is actually a login for mySQL. Somewhere else you must have some info about logging into your host.
For example, if your wordpress domain was http://mywp.com
and your user was admin/mypass, but your site is hosted at http://bighost.com
, you must have a userid/password for bighost.com (lets say it is myname/mypass)
To use your ftp program, you'll have to logon (this can vary per host) to mywp.com, using myname/mypass. This will now allow you to make the changes you need.
I suggest you look at the help files for your host and/or ask them how to FTP to your site. Once you have that down, if you have questions about what fies or where to find the Atahualpa files, feel free to ask here.