You haven't given the full error but I guess you need to replace:
NSData *imageData = [myCache valueForKey:imageURL];
with
NSData *imageData = [myCache objectForKey:imageURL];
You might want to add some protection, like:
if (imageURL != nil) {
NSData *dataToCache = [NSData dataWithContentsOfURL:[NSURL URLWithString:imageURL]];
if (dataToCache != nil) {
[myCache setObject:dataToCache forKey:imageURL];
[cell.imageShow setImage:[UIImage imageWithData:dataToCache]];
}
}