First be sure that portmap daemon is running on your system

$ rpcinfo -p

if not, you have to run it commonly on /sbin/portmap
Then be sure that nfsd is running too

$ ps -A

if not, you have to run it on /etc/init.d/nfs-kernel-server

$ /etc/init.d/nfs-kernel-server start

If you make changes in the export file you have to update nfs-kernel-server with that changes

$ exportfs -ra

Now the nfs server is ready to be use!