Can the apk be side loaded on a Firestick?
Yes just use a program called ADB link follow instructions fairly easy way to download to firestick.
I downloaded this apk and installed it, but when I went to open it it asked me for a username and password.
I made up one and it told me that what i put in was not valid. Today has seemed like a Monday all day so I must have missed some instructions somewhere. Help would be appreciated. How or what do I enter to get to the apk?
You need to join a server of your choice in this case OK3 you will get your user and password etc. Look in the forum here lots of info