i meet the same problem and solve it by:
for example:complie in vs 2005.
right click your solution, choose property-->linking-->general,in the first line" output file" ------DO CHANGE "...\qwt_designer_plugin.lib" INTO "...\qwt_designer_plugin.dll".
and remember all, generate release version,not debug one, or you won't find this plugin show itself in qt designer.
good luck,guys:)