the username which you use in the installation package is the same as the administrative account (administrator in Win7 is disabled by default) that you have created on ALL the machines?
ADNM is mainly build for use in an AD enviroment, it is possible without it, but its a lot more work i believe....
Since the standard scanning for computers on the network is done in conjunction with DNS which is an integral part of Active Directory it is not surprising that you cant find most of the computers, if you use the IP based scan (extra settings) and you still dont see the computers then u probably have a firewall on the client blocking the discovery
(configure the firewall or disable it)
its probably easiest to make a txt file with the computers u got, u dont have that much 1 minute of typing (if u standardised the computernaming ofcourse, which would be best practice) and see what happens from there