To be frank, only a selected few are aware that the Enter and Return keys are not the same – whether you refer to it as Enter or Return, users will press either buttons (if they have both buttons). This is due to the fact that on very few software products the Enter and Return keys do different things (Avid’s Pro Tools is the only example I can give).
My own experience is that Enter is a more popular name; “Press the return key” sounds odd, while “Press Enter” sounds natural. I fail to see anyone to whom you’ll say “Press Enter” going “Hu?”, while I can imagine this happening if you’ll say “Press Return”. But that’s just me and based on my own subjective experience; perhaps in Norway they use Return more than Enter. However, as you said, a search for “Press Enter” on Google yields 15 million results, while “Press Return” yields less than a million.
If I had been you I wouldn’t worry about it so much and use Enter.
The term Return is a shorthand for Carriage Return.
Originally, the term carriage return referred to a mechanism or lever on a typewriter. It was used after typing a line of text and caused the assembly holding the paper (the carriage) to return to the right so that the machine was ready to type again on the left-hand side of the paper.