Hi David - you basically want to click a little ways inwards from the corner on an edge, and then click on the equivalent place on the cylinder.

The key thing is that for the target pick on the cylinder you want to rotate around until you are looking at the "seam edge" of the cylinder which is what you want to map to the left-side edge of the base plane. If you're just looking somewhere around in the middle of the cylinder rather than at its seam area you won't be able to control it properly. So make sure you are near the boundary area of the cylinder when you go to do that pick.

You could choose any of these 4 spots on your base surface:




Let's focus on picking this one on the bottom edge:




So note there the location is towards the left side of the bottom edge. Note that you do not want to pick too exactly in the corner because a pick right exactly in the corner is ambiguous whether it belongs to the left-side edge or the bottom edge, you want your pick to be clearly closest to just one particular edge.

So then for picking the target surface you need to go to the corner of the cylinder surface (which means rotate around so you are looking at its "seam edge" area, and then do a similar pick on the bottom edge of the cylinder, a little ways along it, that would be here:



That will then align the bottom left of the plane to the bottom left of the cylinder, making this result:




You don't have to pick exactly on the edge - somewhere nearby the edge but actually slightly interior to the surface can be easier especially if you're trying to align a left or right-side edge with the cylinder's "seam edge" - if you're doing that one you want to pick just a little bit towards the inside of the surface and not exactly on the seam edge itself because otherwise that's another ambiguous pick since the seam edge contains both the left-side and right-side edges of the cylinder right on top of each other. Basically the cylinder surface is like a sheet of paper rolled into a tube if that helps explain it.


Basically the surface-to-surface flow works by mapping from one surface's rectangular 2D space into the other surface's 2D space and the pick points control how to align the 2D paper sheets with one another. The 2D space is also called UV space with one direction being the U direction and the other one being the V direction. These UV directions are not constrained in any specific way to relate to world coordinate directions, it's easy for example for a plane surface that's flat in the Top view to actually have it's U axis going in the world y axis direction and not necessarily in the world x axis direction and stuff like that. So because of that it's not very good to just directly map between each surface's own natural built in U and V directions, that's why the Flow orientation is decided by pick points instead so that the base surface and target surface U and V directions are reversed or flipped as needed so that the pick points between the 2 are aligned with one another.


There's also a screencast video showing this kind of plane to cylinder flow in action here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5761.3

And also some other discussions here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4795.10
http://moi3d.com/forum/index.php?webtag=MOI&msg=4363.124
http://moi3d.com/forum/index.php?webtag=MOI&msg=4363.65
http://moi3d.com/forum/index.php?webtag=MOI&msg=4442.24


Hope this helps, please let me know if you are still stuck.


- Michael