SynthEdit comes in two main flavours, 32-bit and 64-bit. 64-bit introduced several new technologies including "cross-platform graphics". i.e. graphics that can run equally on Windows or Mac OSX. This poses a problem for loading VST plugins into SynthEdit because in the 64-bit version it is not possible to "embed" Windows-specific graphics. i.e. you can't display the "child" VST plugins as part of your User-Interface.
Secondly, using other people's VST plugins as "child" plugins within your own creation is a legal grey-area and also difficult to support when distributing your creations, because you also may need to provide installers for the "child" plugins. For these reasons support for loading other VSTs into SynthEdit was dropped from the 64-bit version.
Note: It is possible in theory to write a "wrapper" plugin to convert VST plugins to run as SynthEdit plugins (aka "SEMs"). (One limitation would be that the "child" plugins User-Interface would "pop-up" in a new window, it could not be integrated into the plugins main window). However, SynthEdit Ltd is not promising this feature at present. Other 3rd Party developers may choose to do so.