I searched the Internet and found this download hyperlink to the MUGEN stage you mentioned:
I put VP_AtlachHD.def in version of mugen1.1a4 and it loaded up without the error message you got:
But I needed to add a line "localcoord = 320, 240" in the [StageInfo] part of the VP_AtlachHD.def to avoid unwanted stage mis-scaling and mis-positioning with my mugen1.1a4 that uses default setting (i.e. no custom add-on's and no change in mugen.cfg setting).
You make sure you have put VP_AtlachHD.def and VP_Atlach.sff into your MUGEN1.1 stages/ folder?