You would have to put all DLL's that the plug-in DLL depends on into the same folder as the installer EXE (aka $EXEDIR) - not
into the folder from where the plug-in DLL is loaded (aka $PLUGINSDIR). For details look at the Dynamic-Link Library Search Order
. Anyway, I would recommend to link everything statically
into plug-in DLL to avoid dependencies. Qt can be built as static lib. Check your plug-in DLL with Dependency Walker
to make sure it doesn't have any dependencies that could prevent it from loading correctly...
(BTW: The dependency on the Visual C++ Runtime libraries is another problem. Thus I would recommend to build plug-in DLL's with the static
Runtime. Either that or link them against the VC 6.0 Runtime library, aka MSVCRT.DLL, which ships with Windows)