I am getting this error message ("The MD5 signature could not be verified.") when trying to use a FuturePay installation (sending an MD5 hash as the signature field). If I try to send the form without the signature tells me:
"The installation ID field is blank or contains invalid characters.
A different submission protocol is required. A more secure submission may be required.
The installation number is invalid."
I have to mention that I tried to use the signatureFields field (I am getting the same error) and that for the normal Junior installation it works like a charm.
I also must say that the way WorldPay presents the installations is very confusing. While they advice "A more secure submission may be required" when not using the MD5 signature there is no place in the admin area to set one.
I am still assuming that it's the same that you set for the Junior installation.
Double check that you are using the correct type of installation, I believe there are two types of FuturePay - one for website integration purposes and another which can only be used via the WorldPay dashboard.