If you want to add a tile to say tile (x,y) ( (x,y) in Tiled Map editor coordinates ) then use the following code -
myTileMap is the reference to the CCTMXTiledMap object.
CCTMXLayer *layer=[myTileMap layerNamed:@"yourlayer"];
NSAssert(floorLayer !=nil, @"Ground layer not found!");
CGPoint tileAddPosition = [layer positionAt: CGPointMake(x,y)];
//Create your CCNode or CCSprite or whatever...say object name is **tileToBeAdded**
tileToBeAdded.anchorPoint = CGPointZero;
tileToBeAdded.position = tileAddPosition;
[myTileMap addChild:addedTile z:1];