So you have this error:

Command: running       stdout: yes           stderr: no

Before command completion, additional instructions may appear below.

0042-172 NIMkid: This machine's CPU ID does not match the CPU ID
        stored in the NIM database.

What to do in this situation?
Go to the nim master and check if the CPU ID is ok for this client.

You can get this value from the client with prtconf or uname -a:

gznimcl1:root:/home/root # uname -a
AIX gznimcl1 1 6 [color=green]00C34XXXXC00[/color]

[gznimserv:root:/home/root:] lsnim -l gznimcl1
ty7a0221:
   class           = machines
   type            = standalone
   connect         = nimsh
   platform        = chrp
   netboot_kernel  = 64
   if1             = ent-Network1 gznimcl1 0
   cable_type1     = tp
   Cstate          = ready for a NIM operation
   prev_state      = currently running
   Mstate          = not running
   [color=red]cpuid           = 00CE3YYYYC00[/color]
   Cstate_result   = success
   installed_image = mksysb_61-04-01

So the CPU ID is wrong. Modify the nim client via smitty or watever, and after that you should be ok:

[gznimserv:root:/home/root:] lsnim -l gznimcl1
ty7a0221:
   class           = machines
   type            = standalone
   connect         = nimsh
   platform        = chrp
   netboot_kernel  = 64
   if1             = ent-Network1 gznimcl1 0
   cable_type1     = tp
   Cstate          = ready for a NIM operation
   prev_state      = currently running
   Mstate          = not running
   [color=green]cpuid           = 00C34XXXXC00[/color]
   Cstate_result   = success
   installed_image = mksysb_61-04-01

    Configure Network Installation Management Client Fileset

Type or select values in entry fields.
Press Enter AFTER making all desired changes.

                                                        [Entry Fields]
* Machine Name                                       [gznimcl1]
* Primary Network Install Interface                  [en1]                     +
* Host Name of Network Install Master                [gznimserv]

  Hardware Platform Type                              chrp
  Kernel to use for Network Boot                     [64]                      +
  Communication Protocol used by client              [nimsh]                   +
  Ethernet Interface Options
    Network Speed Setting                            []                        +
    Network Duplex Setting                           []                        +
  Comments                                           []

  Alternate Port Numbers for Network Communications
       (reserved values will be used if left blank)
    Client Registration                              []                        #
    Client Communications                            []                        #

Command: OK            stdout: yes           stderr: no

Before command completion, additional instructions may appear below.

0513-044 The nimsh Subsystem was requested to stop.
0513-059 The nimsh Subsystem has been started. Subsystem PID is 3645616.