Do you have a keyboard and a display attached to your raspberry pi? Or perhaps a touchscreen?
In that case, the username is “nymea” and the password is “nymea” too. No need to insert the IP. The instructions Bernhard gave you are meant to connect remotely from a PC to the Raspberry Pi because most people don’t have a display/keyboard attached to it.
However, I doubt that this is actually what you want because all you can do there is some really advanced configuration tuning. Instead, after having the Raspberry Pi installed, just use nymea:app from your mobile phone (install it from Play Store/iTunes Store) and connect to your Raspberry Pi from there.
Another option would be to try our new prerelease community image which comes with a UI when there is a display connected. Then you can use the complete nymea system from the Raspberry Pi without the need of a mobile phone. You can find it at https://downloads.nymea.io/images/rpi3/nymea-18.04.2-pre4-nymea-armhf+raspi-ext4.img.xz
Hope this helps, please let us know how it goes.