To configure your account on Zoiper for Android just go to Configuration -> Accounts -> Add account -> SIP/IAX -> Select yes if you already have an account -> Manual configuration and fill the required settings for Host, Username and Password. If any additional settings (STUN, Outbound proxy, etc.) are required, fill them as well.
As for Error 404 it is returned when the number you are trying to dial is not correct. If you are receiving this error for the account, you would need to contact your VoIP provider for more information on that matter.