Hi Liana,
First of all XyGrib and zyGrib are not the same product. I can speak for XyGrib but not for xyGrib.
XyGrib recognizes the records for Primary Wave and shows their values in the data table on the left of the window. However, XyGrib is not currently showing arrows for Primary Wave although it is possible to choose this option in "Sea->Wave Arrows". This is a bug and I will open an issue on the development site.
I tested a FNMOC-WW3 grib with records for Primary Wave Direction and Primary Wave Period with not other records and the arrows are shown in zyGrib. As mentioned above they do not show on XyGrib. I can only suggest that you visualize in zyGrib until we fix the bug in the next release.
Best,
David