[Freesurfer] scatterplots in tkmedit

Hello, I ran a GLM using a 4d skeletonized Fractional Anisotropy volume as my input into mri_glmfit with one variable. As this analysis is not surface based, I must open the results (sig.mgh) using tkmedit as an overlay. My goal is to open a scatterplot similar to that in tksurfer which shows the voxel value vs. variable score for each individual subject at a particular location. When I try to load the GDF file (the y.fsgd output in glmdir right?), it also asks for a registration file. I don't think there is a register.dat file as an output in the glmdir.... so, I was wondering what registration file it is looking for?

Thanks for the help! Tori