The solution depends on the OS which your probe is running on.
For Windows, when the probe was installed as a service it would has a default password of 'killer', you will need to supply this.
For Unix, unless you specified a password to use, it will have a password of null, you cannot specify this, so it will need to be set (see below)
If you installed the Unix probes with the -nopassword switch then it would not check for a password.
A solution is either to start the probes with the -nopassword switch, this is the simplest solution if you aren't concerned about passwords. Alternatively, under the probe advanced tab you can specify an 'encoded password' which will override whatever currently exists or in the case of a null password replace it with whatever you specify.
Another option if you are trying an e-mail script, is to run it from the gateway rather than the net probe binary.