How do I use curl to download content from sourceforge?

You can do

curl -L  http://downloads.sourceforge.net/project/romfs/genromfs/0.5.2/genromfs-0.5.2.tar.gz > genromfs.tar.gz

to download the file.

The -L tells curl to follow any redirects, which sourceforge normally does.

If wget is available, that would be far simpler.

You have to use -L option, because sourceforge uses redirects to the download location. Try -v switch and you will see headers of requests and answers.