I have a Sony STRDN1080 and a Mission 78C centre speaker. The speaker does not have crossovers and has four connectors and needs to be biwired. How should I connect the speaker to the amp as the amp only has two connections for the centre speaker?
Biwiring is pointless without also bi-amping and the STRDN1080 has no facility to bi-amp the centre speaker. You are simply doubling the wire thicknrss between the receiver and the speaker if bi-wiring without bi-amping and you can do this without bi-wiring by simply using thicker wire. Besides this, there's no advantage to using thicker wire unless you are conveying a signal over a considerable distance.
The Mission 78c is biwirable, but does include an integral crossover. You don't have to bi-wire it. Just leave the bridgeing plates in place and wire normally. If the plates are missing then you can bridge the connectors using speaker wire.