As David states, the context menu for f(x)=2*x+2 does not include Plot. Could it be possible that you had entered f(x):=2*x+2, and the typesetting disambiguator turned this into f:=x->2*x+2? I'm guessing that's what you did, as the output you got would be consistent with this situation.
First, f(x):=2*x+2 does not mean what you might think it means in Maple. In 1-d math input, this normally defines f as a procedure with a "remember table". I won't distract you with the details but you can look at the 'remember' help page if you're curious.
When you enter f(x):=2*x+2 in 2-d math in the worksheet, Maple thinks that you probably meant to write f := x->2*x+2, as use of remember tables is not common, and it asks you via a dialog box whether that's the case. This actually defines a procedure f which then returns the value 6, for example, when you call it by entering f(2).
The plotting commands work on both expressions such as 2*x+2 and procedures such as x->2*x+2. However, the zoom feature which David describes works only when the input is an expression and not when it's a procedure. As a side note, keep in mind that in Maple, f(x):=2*x+2 (an assignment) is not the same as f(x)=2*x+2 (an equation).