First off Dutch, did you verify that imageMagick is working according to the last couple of paragraphs on this page?
ImageMagick: Install from Binary Distribution
Also, to quote that page:
"If you have any problems, install the
Visual C++ 2008 Redistributable Package (x86) or
Visual C++ 2008 Redistributable Package (x64).
64-bit Windows requires both packages (x86 & x64)."
It sounds like you verified imagemagick but we need to be sure.
After adding the plugin did you do this step from this page
LR2/Mogrify - Add Watermarks, Border and Text Annotions to Images Exported by Adobe Lightroom 2 ?
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Step 4: Set up (Windows users only)
Windows users now need to tell the plugin where to find ImageMagick's
mogrify application.
- Go to Lightroom's Library module, click on an image and then press the Export button.
- On the left side of the export dialog box, under 'Post Process Actions' double click on 'Mogrify Configuration'.
- In the Mogrify Configuration panel which is now showing on the right, press 'Choose', browse to the ImageMagick installation (e.g. C:\Program Files\ImageMagick-6.4.1-Q16 - this will vary with the version of ImageMagick) and choose mogrify.exe

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If none of this works go to the export panel in LR and click on the Plug-in Manager button on the lower left corner. Then highlight LR2/Mogrify in the left hand box if there are more than one plug-ins. Then in the "status" box thingy on the right, where it shows the path to the plug-in, click "show in Explorer". In that folder I have a info.lua file, let's check to see if yours is there.
I have no idea why it may not be, maybe a bad download? I can send you a copy of mine or maybe try re-downloading Mogrify again and seeing if the file shows up.
If none of this works we'll try something else:-). Sure is a convoluted way to get something working, no doubt.