In single-user mode it always mounts the partitions as Read-only.
But to be able to mount it as read/write type:
mount -uw /

Sometimes if the partition isn't clean it will tell you to use the fsck
command then you can mount the partition.

Hope this helps 
> Ok, I downloaded NetBSD onto my Performa 600 and have been trying to
> configure it for two days.  I'm no UNIX Admin, but I'm going to learn very
> quickly.  But here is my current problem.  I can enter in single-user mode
> and everything seems to be where it is supposed to be.  But when I try to
> run vi, ex or a multitude of other programs, it keeps saying that this is a
> Read-Only File System.  Am I missing something big configuration-wise?
> This happens with Booter 1.9.4 and NetBSD 1.1 or 1.2.  I've got it set up
> with a Root/Usr partition of 80000K and a Swap partition of 40000K (I have
> 20 MB physical RAM).  TIA