Error with custom UICollectionViewCell

I had this exact issue yesterday (as the documentation says you MUST register the class), but the documentation is inaccurate in that you should not register if a prototype cell exists in your storyboard and you give it an identifier.

If you create a prototype of the UICollectionViewCell in your storyboard and assign your custom cell class, it is not necessary (and is detrimental if you do) register the custom UICollectionViewCell. This is the reason the resulting UICollectionView is black.

Based on the error message, the prototype cell in the storyboard does not have its identifier set correctly (either not set at all or is not MediaCell)

You're right in using the line

[self.collectionView registerClass:[MediaCollectionViewCell class] forCellWithReuseIdentifier:@"MediaCell"]; 

as this helps with cell reuse.

The problem could be your cellForRowAtIndexPath method, or your subclass, or the way you set up your delegates. Can you post the code?

Your delegate method should look something like this:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    MediaCollectionViewCell* cell = [self.collectionView dequeueReusableCellWithReuseIdentifier:@"MediaCell" forIndexPath:indexPath];
    // set cell.image
    return cell;
}

You should set up your delegate in your viewDidLoad method with...

[self.collectionView setDataSource:self];
[self.collectionView setDelegate:self];

EDIT: Can you post the code for your MediaCollectionViewCell implementation? I did something similar, and mine looks like this.

- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self) {
    // Initialization code
    self.imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, frame.size.width, frame.size.height)];
    [self.contentView addSubview:self.imageView];
}
return self;

}