I had the same problem.

My solution was you need to copy 2 files to separate locations where qt can find them:

qwt.dll and qwt_designer_plugin.dll

On my machine this was: C:\Qt\4.6.3\plugins\designer\qwt_designer_plugin.d ll
and C:\Qt\4.6.3\bin\qwt.dll

Then if you are using designer embedded into qt creator I copied to one more 3rd location: C:\Qt\qtcreator-2.0.0\bin\designer\qwt_designer_plugin.dll

I don't know if this is what is intended %100 by QWT but it worked for me on windows.

Luck,

Seth