Tudy,
Are you using more than one Monthly Archive? You can look in the weblog config under archiving to find out.
If not, then you should be able to use
CODE
<MTArchiveList lastn="1" archive_type="Monthly"><a href="<$MTArchiveLink$>">Archives</a></MTArchiveList>
That will give you a link to the latest Monthly archive.
If you are using more than one Monthly archive, what are you using the for the Archive Filename on the Calendar Archives? The same tags in the Archive Filename can be used to link to that page.
Hope that helps,
Kristine