Are you using Band In A Box or RealBand? Either way by 'real instrument' i presume you mean a RealTrack. RealTracks are recordings of real musicians playing live riffs and pharsaes that BIAB stitches together to give you a recording of a real musician playing along with the chord changes. but you are confined to phrases and riffs the musician recorded. you can't ask it to play a melody.

how are you trying to play a melody at the moment? yes you can get a solo played on the chord changes. but if you want to play 'the melody' you'll have to use midi. do you know how to work with midi? and do you know what midi is as opposed to audio recordings?

before anyone tries to help its worth letting us know if you are familir with midi. apologies if all this is stuff you know but the question you ask is a bit confusing. a 'real instrument' is what you get with a RealTrack but as I say, its limited to what has already been recorded.