achievements-cell.m

// get current achievement from array as a description
GKAchievementDescription *achievement = (GKAchievementDescription *)[self.achievementsList objectAtIndex:row];
// set the cell's text to the title
cell.textLabel.text = achievement.title;

// check to see if achievement has been completed
if ([self.achievements objectForKey:achievement.identifier])
{
    // get the achievement from the dictionary using the identifier
    GKAchievement *theAchievement = [self.achievements objectForKey:achievement.identifier];
    // more code...
}