For starters, I *am* able to connect to your FTP server.
I first need to know about your internet connection. I don't need to know what kind of computer you have, but I do need to know how it connects to the internet. If you have a router, I need all the details about that.
I would also like you to try a few things.
First, try a command-line ftp program. These are not hard, and there is one included with Mac OS X. Simply open a program called the "Terminal", which is in your Utilities folder (inside the Applications folder). When it opens, try the following:
1) type 'ftp web78.opentransfer.com' and press return. (Don't type the quotes in any of these commands!)
You will be prompted for username and password. Satisfy these credentials, and try two commands. First type 'ls' and press return. That should return a list of everything in the directory. The other thing to try is uploading a file. Type 'bin' and press return. Now type 'put' and press return. The program will ask you for the 'local file'. Just drag a file from the finder into the terminal. Now, in the terminal, press 'return'. The program will ask for the name of the remote file, so think of a name for it, and press return. When you finish this task, use the quit command to exit.
Chances are, some of that didn't work, so copy it all down, and paste it into a message here.
The second thing to try is to completely clear out your firewall rules. Sometimes, a local firewall will prevent the server from opening a data connection into your computer, for the purpose of transferring files and other information.
You can accomplish this with a single command in the terminal. Open a new terminal window (apple-n in the terminal program), and type this command: 'sudo ipfw flush'. (This assumes you are logged in as a user with administrative privileges, which I suspect you are.) sudo will prompt you for your password. Type it in, press return. ipfw will then ask if you are sure, so type 'y' and hit return.
After flushing out the firewall, attempt the file transfer again. Try both your normal ftp programs, as well as the ftp program that runs in the terminal.
Hope this helps!