Implement the method "plus"
First off: Apologies for the shortness of this review. There's not much code, after all.

You've got a bit of inconsistent style with whitespace after your cases -- I haven't done Scala in a while, but if I'm remembering right, you're supposed to have a space after.

I'd recommend renaming i and s to i1 and s1, respectively, just to be consistent.

Your 'spec' says that "a" plus "b" should return "a and b = ab", but it actually returns "a and b". Personally, I think it should return "ab", a la string concatenation.

Using Any is not a code smell, though some error throwing may be in order if any types other than Int or String are used, so that if I call 1.0 plus "a", I don't just get nothing. It's up to you which type to use, though personally I'd use an IllegalArgumentException.

Aside from that, it looks good! Well done.