Musescore won't allow me to add new symbols to my palletes

I was writing a work for my composition class and wanted to use a mezzoforte piano (mfp), which isn't in the dynamics pallete. I went to the pallete to add a new symbol, right clicked and the more elements option appeared in gray, so, I can't add any symbols because the option doesn't appear as active. Please help.

Thank you.

GIT commit: 3543170