where did you get the software you're trying to install? all of the student versions have different levels of access.
also, with your serial, it should know what you're entitled to when you connect to the internet, so even if you don't enter it in the right boxes, it should install the right components. if it isn't, my guess is your version does not have flow.
I got the software from thinkedu which is a large distributor of educational software. It was listed that flow simulation was included in the software. Here is the link for the software I got: http://store.thinkedu.com/solidworks-student-edition-2012-13-wsolidworks-essentials-training-videos--manuals-p519.aspx
I was only given one serial number, so I am not sure what to do.
there should be one serial
if you're connected to the internet when you install, it should realize all the components that you're entitled to
you can try entering it in the solidworks, simulation and flow sections but i don't think it will do anything for you
just put it in the solidworks section
then on the next screen, there is an option that says programs to install, show programs that aren't included, make sure that simulation and flow are selected
finish the install
start the software, activate
look at whether it shows solidworks, flow and simulation
if it does, you're good
if it doesn't, call thinkedu or solidworks