The only thing with adding them manually is that if the other sites know this, I'm not saying they would, but they might for some reason take your link off their website - and you would never know (especially if you have hundreds of links), you just wont be able to keep track.
Then you have the situation that you wont move any further up google cos you dont have links on other sites, but you are promoting nicely OTHER sites. Then all your hard work is wasted.
I'm not saying it could get like this but you never know! Percervere with it, Im glad I got ours done and if you have more than one site it can do up to 3 (but you have to pay about £5 for each extra one). This is good though cos you only have to set it up the once