On a dedicated NetBSD system on standard PC hardware, NetBSD is started by the code in the first sector of the diskdrive, the MBR. This code scans the partition table for an active partition and passes control to the boot code (volume boot record) at the beginning of this active partition.
See
http://en.wikipedia.org/wiki/Master_..._bootstrapping
In most boot managers you will only have to select to the MBR partition number of your NetBSD install.