Chiashurb,
When making melomels from juices or juice concentrates, there are many different methods that people follow. Some only add the juice in primary, others wait for secondary, some do both, some actually wait until bottling time, and some use whatever technique it takes to achieve the effect they're after.
As a rule of thumb, fruit and fruit juices fermented in primary will change in flavor -- for instance, fermented grape juice (wine) doesn't taste much like unfermented grape juice. This is not to say that this is a bad thing -- it simply depends on your taste. Juices added in secondary fermentation will usually hold a closer bead on the original fruit flavor. Some folks add fruit and/or juice in both primary and secondary in order to achieve a "layered" effect with the flavor. It's whatever you're after.
As for pasteurizing fruit juices, this is not generally necessary, and is even counter-productive, since virtually all fruits and fruit juices will have pectin in them to some degree or other, and heating makes this set, the way grape juice will become jelly. The resulting mead will oftentimes gain a pectin haze that will be very hard to remove, though it doesn't effect the flavor. Still and all, some people boil or heat their must (in this case, honey, water, and juice together, or just honey and juice), and add something called pectic enzymes, available from any homebrew supply store. This has the effect of breaking down the pectin's tendency to set, and, in theory, will produce a nice clear mead. I've heard of mixed results with this, actually, and I generally use pectic enzymes to solve particular problems, rather than simply as a matter of course. And, again, if you don't heat the juice, you won't have the problems to begin with.
-David
