<p>I am trying to add a <code>UIButton</code> at runtime however it is not visible. What am I doing wrong?</p>

<pre><code>- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        UIButton *btn = [[UIButton buttonWithType:UIButtonTypeRoundedRect] retain];
        btn.frame = CGRectMake(0, 0, 100, 25);
        btn.backgroundColor = [UIColor clearColor];
        [btn setTitle:@"Play" forState:UIControlStateNormal];
        [btn addTarget:self action:@selector(buttonClick:)
        forControlEvents:UIControlEventTouchUpInside];
        btn.center = self.center;
        [self addSubview:btn];
    }
    return self;
}
</code></pre>

First, make sure the initWithFrame: method is being called. If your view is in a Nib, initWithCoder: is being called instead.
Second, is the button the only subview (from your code it looks like it is, but you never know). The button could be hidden behind another subview. Call bringSubviewToFront: if you need to.
Finally, is the view itself visible? Is it big enough to show the button? Given your example, if the view is less than 100 pixels wide, the button won't show because it will get clipped by the view's bounds.