I tried to access the admin webpage http://dcerouter/pluto-admin
I tried user: linuxmce password: linuxmce (which is the default, I haven't changed anything)
but it doesn't accept. Do I need to login with a different ID?
The user for webadmin is set during install.
It depends on the name you set for your first user.
By default your password is the same as your first users name with the with one difference. First letter is replaced with a capital one.
User linuxmce is only valid when you access your system from console or through ssh.