Arcues is pronounced
AR-SEE-US in the anime. Ash and his companions always pronounce it like that, so technically it isn't the wrong way to pronounce it.
However, according to the Pokemon: The Essential Handbook, it is pronounced AR-KEE-US.
So, to be honest, it really depends on how you'd rather say it. I say it as Ar-see-us, because I think it sounds better, but I know people who say it as Ar-kee-us.
Source: The Pokemon Anime and Pokemon, the Essential Handbook.