Composing a Melody For an Instrumental
My advice is lay your basic tracks and if nothing comes to you melody-wise immediately, live with the tracks for a few days. I also suggest that maybe you don't ask people for feedback -- at least immediately.
Anyway, put the tracks on a CD or an MP3 player and listen to them for a few days. Here's the key: the composition SHOULD be able to stand on its own merits without a melody track. If the basic tracks don't excite you -- if they don't get you rockin' or give you that yeah, baby, or the fist-in-the-air thing, no melody is going to save it anyway. If this is the case, the song needs work. Go back and fix that.
But let's say for a moment that you really like the composition and you're stuck on the melody. The same idea still applies. Put them on a CD or an MP3 player and listen to your basic tracks for a few days. If nothing comes to you naturally (sometimes it will, sometimes you have to work for it) you have to ACTIVELY listen and see if you can imagine a melody of the part. Do this by just humming it, -- don't have your guitar in hand. Your brain has heard zillions of melodies and songs in your lifetime and is probably not bound by the comparatively small amount of scales modes and theory you know on guitar. You may find you'll hum in a scale or mode you rarely play in or don't even know. That's a GOOD thing. But once you hear the part in your head, and if you can hum it, you should be able to figure out how play it.
Sometimes it's good to use a small cassette recorder to record your ideas. For example, let the basic track run and hum your melody over top, then go back and learn to play the melody you hummed.
If none of that works, start by playing the notes that comprise the underlying chords. Play them randomly, connect them at will, sometimes that will get you started and then you can connect the dots with the other notes in the scale.
And just because you're looking for a melody doesn't mean you can't employ some basic lick-techniques to get you kick started. The double-stop, the unison bend, ocatave runs (with wah), vibrato, tremolo picking etc -- these devices are your friends, and can help get you started. THIS IS WHERE ATTITUDE LIVES! THIS IS WHERE THE BALLS COME FROM IN SOLOS. A melody can be sexy, happy, sad, haunting, etc. but usually not ballsy. Balls isn't an emotion, it is the attitude with which you play.