there's some things going wrong here.
First, you shouldn't be adding other parts into the baseplate itself.
Instead, you need to move my mpd file to e.g. your "models" folder.
Then create a new file, and insert that part there.
Then you can add more other parts.
What I see from your screenshot is that you've inserted more parts _into_ the *.mpd file,
this is not how you're supposed to be building with LDRAW.
Said that, I think that MLCad 3.40 may have a bug here.
What I see from your screenshot is that the added part gets scaled _up_ by MLCad...:
it has an option at "Settings/General/Change.../Document/Orientation of new object"
which says "same as previous". This seems to go mad here. MLCad seems to insert
the matrix of some line of the MPD as default here. Which of course will hurt badly for parts.
They're not intended to be scaled up, just rotated and offset.
Try if setting the option to "Default" heals the upscaling.
Nevertheless, you should be following my advice above: do not _insert_ other parts _into_ this part,
but instead create a *.ldr file (a "scene"), in which you insert the mpd _plus_ the other parts.
First, you shouldn't be adding other parts into the baseplate itself.
Instead, you need to move my mpd file to e.g. your "models" folder.
Then create a new file, and insert that part there.
Then you can add more other parts.
What I see from your screenshot is that you've inserted more parts _into_ the *.mpd file,
this is not how you're supposed to be building with LDRAW.
Said that, I think that MLCad 3.40 may have a bug here.
What I see from your screenshot is that the added part gets scaled _up_ by MLCad...:
it has an option at "Settings/General/Change.../Document/Orientation of new object"
which says "same as previous". This seems to go mad here. MLCad seems to insert
the matrix of some line of the MPD as default here. Which of course will hurt badly for parts.
They're not intended to be scaled up, just rotated and offset.
Try if setting the option to "Default" heals the upscaling.
Nevertheless, you should be following my advice above: do not _insert_ other parts _into_ this part,
but instead create a *.ldr file (a "scene"), in which you insert the mpd _plus_ the other parts.