I have set up a few NLB clusters without problems, so far they have been administered with the administrator user.
Now I have created a few users who are identical on both members of the cluster, all with administrative rights. When the users try to start the NLB manager they get Access denied when the manager tries to the other member. This works perfectly with the Administrator account.
If this is a windows 2008 server, this is a known issue. You'll need to disable UAC and reboot both servers for this to work.
We ran into the exact same issue.
There is a technet blog describing the issue as well.