Well if it doesn't have the tab on the connector, something is wrong e.g. it's a fake or someone has messed around with it.
That said; plugging the servo in backward will not hurt anything. The hot lead on all servos is in the center with the signal and ground lead on each side. So if you plug it in wrong, it just won't work. If you did the same thing twice and it doesn't work now, check the pins in the rx, make sure you didn't bend one. Are you using a good battery with the appropriate voltage? If either the rx or the servo are clones, one or the other may very well have quit.
May I suggest that when you are new at this, buying your radio gear at a local hobby shop who can help you out and offers the warranty on new parts with no issues about buying fake/clone stuff which is really prevalent on eBay.
Last edited by Zeeb; 07-22-2016 at 11:03 AM.