Pelvis Converter PlugIn needs Vicon PlugIn Gait - or at least the pelvis progression angles generated by PlugIn Gait in the trials you want to process.
The Pelvis Converter PlugIn converts the pelvis progression angles generated by Vicon PlugIn Gait from a YXZ floating-axis Euler angle, to a ZXY floating-axis Euler angle. It does this by re-composing the Euler angle from PlugInGait into a 3 x 3 rotation matrix, then decomposing the matrix again, but in the required rotation order.
The results are saved in two "angle" trajectories in the trial. The names of the two trajectories used (left and right versions are generated) can be specified in the settings dialog. The default names used are LNewPelvisAngles and RNewPelvisAngles. These names have been added to the accompanying ConvPelvPiG.mkr file (a modified version of PlugInGait.mkr) which may help when analysing your results in other software such as Vicon Polygon. Please refer to your Vicon documentation about how to use this new file.
First of all, there's nothing wrong with PlugInGait. It's just that the Euler angle representation it uses is not so clinically intuitive as that suggested by Richard Baker (and others). The fact that the Pelvis Converter is able to perform the conversion using the PlugIn Gait Euler angles, proves that in fact, this gives a perfectly valid representation of the orientation of the pelvis.
Pelvis Converter uses the PlugIn Gait angles, rather than calculating them directly from the marker data, because the progression frame calculation in PlugIn Gait is a relatively complicated algorithm. This appoach guarantees that the two calculated pelvis Euler angle triplets will always agree, even if things change in PlugIn Gait in the future.
If the two new angle trajectories do not appear as you expect, then please check the Workstation Processing log (in the View menu) to get feedback from the PlugIn.