It's probably talking about the demo zip file which you can download from their website. Just go in , click on max2mdl , then on the right click on the demo , and downloado it, then click on the manual , and follow the installation instructions to install the demo , then just copy the key into the installation folder. In fact , here are the instructions from the manual (by the way , when it talks about the zip , it's talking about the max2mdl.zip which is the demo you download from their site).

Installation of max2mdl is a breeze: just unzip the max2mdl.zip file into the
scripts\startup folder in your 3dsmax# folder (e.g. c:\3dsmax6\scripts\startup). You
should end up with a folder named max2mdl with the max2mdl.mse file and a few
additional files.
Additionally, after you buy the full version, you will receive a file named
max2mdl_key.zip; just unzip this file in the previously created max2mdl folder. And
that’s it! Next time you start 3ds Max, you will no longer have the limitations of the
demo version
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