Connection problem can be due to an incorrect path. Please check the path which you have provided.
To find out if the problem is with Mac or with Lightroom try this: Export your web gallery to your hard drive and upload it with a stand-alone FTP program - for instance File-Zilla.
FileZilla is quick to download and very easy to use:
Here are a few threads with similar problems:
And also, check and re-check your FTP data (address, password, etc).