I guess they may be recompiled with the new SDK - assuming Ahead still support a plugin development SDK in V6 ( it would be a huge step backward if they didn't )
If V5.5 and V6 are that different though, it would probably mean 2 versions - unless the V6 SDK is backward compatible
Just looked again, refreshed, and it says they've been tested with Nero 6.0.0.0
Do you have the May 12/13/16 versions from here (versions 1.0.0.26 / 27)
http://neroplugins.cd-rw.org
If you have an "ALL" bundle from there, it must be too old - as the number of plugins increased, they stopped the "ALL".