OK, checked mikeseith's files (it's easy, you can drag-and-drop them on Model Tweaker all at once, not having to open them one by one), and all but "renault dealer truck.dat", indeed cannot be opened in the Tweaker.
Model Tweaker should be able to open all valid dafiles. The error-messages you are getting are because of some sanity checks performed by the application on loading a datfile. Model Tweaker refuses to open datfiles that don't pass these sanity checks. It won't attempt to fix them, because it's not a general-purpose modding tool. Erroneous datfiles should be fixed with the Reader first. And this is what the message actually says, ie "Please fix this archive using the Reader", rather than "Use the Reader to modify your models".
The sanity check that causes the problems with mikeseith's cars is the DIR file (it must contain the same number of entries as the compressed files in the archive - if not the message is displayed). Mikeseith's plugins contain TWO DIR files though, which is wrong. Most probably he developed his plugins separately and finally merged them, in a wrong way though, ie copying ALL files (incl the DIR one). Tweaker doesn't make a check for this condition (duh, I may put this one too in some future version, but how could I have guessed?). Instead it just happened to find the erroneous DIR file, so you got the message.
The solution is quite simple (though a little tedious, for so many files): Open the file in the reader, delete all duplicate DIR file(s), so that there's only one left (it doesn't matter which one, because the Reader rebuilds the DIR on saving), and save. I would say use DatPacker to merge all these datfiles into one (it will leave only one DIR file), but this wouldn't be very much helpful, because mikeseith's models have names like "1E010400_$$$planter-concrete3_Flora2x2x5_Z5S", yes! Talking about the automata, props are named differently, not in a very helpful way either though. But then there's another problem: how are you going to resize them? They won't be fitting into parking lots nicely.
Hope this helps.