Here's a suggestion though that might work. Not sure why you'd need to link to the previous entry from the main page unless you were setting up some sort of journal and you wanted the main page to be the latest entry.
If it's something like that, where only one entry is on the page, you might be able to do something like this...
CODE
<MTEntries lastn="1" offset="1">
<a href="<$MTEntryLink$>"><<before</a>
</MTEntries>
I haven't played around with offset yet, but if I understand it correctly, this should provide a link to one post starting one post BEFORE the current post. In other words, the previous post.
Again, if this works, it probably won't work too well if you have more than one entry per page (but if that's the case, you don't really need the before link)