@send2r,
I would recommend taking a look at the User Guide for the RE350v1
If this is a different network and you do not have access to the login information, you will need to perform a factory reset and reconfigure the device to extend your network. After a reset is complete, the login for the extender's settings is admin for both the username and password by default.
If you have already done this and cannot get the device to work, try reconfiguring the device, as REs are infamous for improperly configuring themselves.
If you are talking about the password of the Wi-Fi when connecting, the password will either be the same as your regular Wi-fi network, or it can be set separately from the Range Extender's settings.