how to convert integer to string?

If you really want to use String:

NSString *number = [[NSString alloc] initWithFormat:@"%d", 123];

But I would recommend using NSNumber:

NSNumber *number = [[NSNumber alloc] initWithInt:123];

Then just add it to the array.

[array addObject:number];

Don't forget to release it after that, since you created it above.

[number release];