Umm have you tried taking off the existing flash suppressor? Because I'd imagine if you did you'd be able to see if it has a threaded barrel or not. The same one came on my Classic Army MP5K and then there should simply be a thread protector piece that just screws off.
Standard size is 14mm and usually counter clockwise threads. Some guns like Classic Army (at least on my MP5K) and G&P have CW threads instead.
With MP5's it's also possible to get the 3 lugged flash hider and special suppressors that fit onto it. Haven't seen one for a while but I haven't been looking at MP5 parts for a while.