Magma flow... Hmm... Is this directly associated with "Semi molten rock?" If so, you've not yet hit the liquid that is usable magma.
Did you ever get "You have discovered a magma sea!" or similar alert? If not, then you won't get magma workshops appearing in your job-list. So you need to find a
proper bit of magma sea (or pipe).
(This might also happen if you've used DFReveal to find the sea, and then not unrevealed before breaching, or somesuch. I think the fix for this is to seal off all fort areas with a view of now-revealed magma, use the DFutility that unreveals all inaccessible areas and then re-breach that seal (not necessarily directly
into the magma, but so that you now get a "fresh viewing" of it). But I believe best practice for DFReveal is to use it to look, but then immediately cancel it, so that it doesn't save, or something. Check the DFHack pages for that...)
I'm guessing, though, that what's happened with you is that you've just by-passed
all proper magma, by just drilling down at a point where it happens to be normal rock (on your explorative path and all adjacent tiles) all the way down to the SMR. It's certainly possible to inadvertently bypass one or more caverns, this way, and the magma sea is effectively just another cavern, in many ways.
At some point maybe a at least a dozen or two levels up from where you hit the SMR (safest, though slowest, start again from the surface, or immediate sub-surface) and make another exploration shaft a dozen or two tiles over to one side. You may then hit any caverns that you might have missed, plus the magma sea. (Or reveal a magma-pipe poking out into at least the lowest cavern, but now within sight of your exploration attempts.) If you
still can't find it, well, that's against the odds, but always possible.
(Oh, assuming you haven't customised your worldgen to rid it of the magma-layers, or something, but you should know if you've been messing with that.
)