Hi,
If nothing is uncommon here, ARP table should be 1.3.6.1.2.1.4.35:
http://cric.grenoble.cnrs.fr/Administrateurs/Outils/MIBS/?oid=1.3.6.1.2.1.4.35
If it's wrong, let's download complete MIB of an EXOS switch via Extremeportal (in downloads under desired firmware version) and drill down to see if it's some non-standard MIB for ARP table.
Hope that helps,
Tomasz