Got instructions how to fix a MAC/Yahoo Messenger problem but I don't know how to handle.?

Sorry, new to MAC and I had a problem with Yahoo! Messenger that someone answered nicely. Unfortunately, I don't know how to do what they told me. I asked how to fix a scroll problem in Yahoo! Messenger and got this response:

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edit /Applications/ Yahoo! Messenger.app/ Contents/ Resources/ Default.ymStyle/ main.js (line 417)

body.scrollTop = body.scrollHeight - document.clientHeight;

to

body.scrollTop = body.scrollHeight;

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Sounds like the Windows equivalent of REGEDIT and it looks like I have to edit a line in the .app file but I don't know how to actually do the editing in my new MAC world.

Can someone help? Maybe explain a little about what it is that I'm actually doing?

Thanks!