"make install" also installs the designer plugin - all you need to do is to tell the Creator how to find it. Look at the end of the INSTALL file about how to do it.

A common source of problems on Windows is, when you are using a binary package of the Creator, that has been build with a different compiler than the one you have used for building Qwt ( MinGW, MSVC ). Also you can't load a debug Qwt plugin into a Qt Creator, that has been buit in release mode ( and v.v ).

Uwe