ACS (Action Coding Script?) is a compiled scripting language that works along side with the Hexen map format. The compiled scripts are stored in a map's BEHAVIOR lump in a binary format decoded by the zdoom/hexen engines. The license the ACS interpreter was released under is not GPL complaint, so there is no ACS support in Odamex. Here is some info I dug which can aid in rewriting the interpreter.
http://odamex.net/doc/thirdparty/acsspec.txt - Unspecified author, found in the zdoom utilities source.