Q: I can't seem to find an app for my iMac for Yahoo Messenger. Is one available?

It's old, but I think it still works: Yahoo! Messenger 3.0.2. Also, check the Mac App Store.

How to safely use MacUpdate to download malware-free software:*

 

This site has both free and paid membership accounts. If you have neither then some software will be distributed as an installer wrapper that includes adware/malware you may not want. Such a download may appear on your computer like this: Firefox Installer.dmg. Delete the download and return to the main site where you will find a direct link to the developer's website. Use that link from which to download the software.

 

To avoid such downloads from MU just create a free membership account. Log into your account prior to using the site. This will avoid the installer wrappers and downloading adware or malware. I continue to use their site without any problems.

 

If you prefer not to create a membership account then note that on the download page under the price box will be the link to the developer’s site. Use that link and download the software directly from the developer circumventing the use of MU altogether.

 

*You merely need to establish a free or paid membership to avoid problems getting adware with your download.

OOtherwise can the multi chat client trillian likely do it

You can use the stock OS X Messages app.

Mac Basics: Messages helps you chat instantly - Apple Support

You're welcome