The compiler is complaining that the method parameter doesn't make sense:
public double compareTo(otherMP3.capacity)
You'll want to check out tutorials on how to declare method parameters, but in general you declare it like a variable with parameter type followed by a parameter name. using just otherMP3.capacity doesn't make much sense.
That the parameter type should be MP3 or object depending on whether you're using generics or not, and you will want to call it otherMp3 or something similar. Then in the method body, get its capacity field's value and use it for comparison. Also, this method should return an int, not a double as you've defined it. Also, your instructions state that the class should implement the Comparable, and you will want to do this. If you're using a generic Comparable<MP3> interface, then again the method parameter will be MP3, and if not, the method parameter will be object, and you must cast it to MP3 inside of the method.