How do I validate a client to allow it to
do a bplist or bprestore from another client.

Example:   Machine A (Backups)

I want to restore to machine B:

bplist -C MachineA 

I get:

EXIT Status 135: client is not validated to perform the operation.
Simply do the following:
Unix:
cd /usr/openv/netbackup/bin/admincmd
./bpclient -add -client MachineB -list_restore 3
cd /usr/openv/netbackup/db/altnames
Windows:
cd  $PROGRAM_PATH\VERITAS\NetBackup\bin\admincmd
bpclient -add -client MachineB -list_restore 3
cd $PROGRAM_PATH\VERITAS\NetBackup\db\altnames
Make a file called MachineB and in it have one line that reads MachineA.
The altnames dir may not exist so you will need to create it.