package.path and package.cpath
local mod_path = string.gsub( et.trap_Cvar_Get("fs_basepath"),"\\","/") .. "/" .. et.trap_Cvar_Get("fs_game") .. "/"
package.path = package.path .. ";" .. mod_path .. "luascripts/?.lua"
That is what it looks like in lua. I think this is what you want. (Changing it in in the c api, may be different though im not sure)