This might be a bit late, but just in case:
I assume you have a Arc ASCII file, which is one of the file formats MCK supports. You can't simply convert ASCII file into log file, since log file only contains pointers to raster images of a certain theme. ASCII file contains much more, like headers and the actual data.
You can create a new log file in MCK by choosing New from the File menu in the Menu bar. This opens the Log File Editor, and there you can specify the contents of the log-file with an easy UI. There is an Import button, which allows you to import the ASCII files. If you have downloaded the MCK software, just see the User Guide that comes with it. It has the whole Log File Editor explained!