Workouts that have the goal of adding bulk rather than burning fat are as unique as the people who use them. When losing weight isn't an issue, all of the elements of developing your physique need to involve adding things rather than taking them away. Even your treadmill and dumbbells need to account for what you're trying to accomplish. Use the tips here to help you bulk up rather than slim down.
First of all, it's time to stop wondering why you aren't bulking up. Sometimes even people who lift heavy materials all day long have a difficult time bulking up. In part, this has to do with the fact that your body is used to what you do at work, so it doesn't add or take away from anything, aside from the usual physical issues that come with doing the same things all the time. That being said, you can still use your daily activities to help you out.
Your workout should involve very little cardio. You want to keep cardio as part of your workout plan, but only in an effort to keep your cardiovascular system healthy. That means these workouts should either be limited or should pre-empt strength workouts.
Use more weight and less repetition. If you can lift the weight twenty times with ease, it's too light for your purposes. That might be okay for a warm-up, but it is not going to help you add bulk. Instead, use enough weight (and a spotter) so that lifting it 5 or 10 times is a challenge. Be sure to use a variety of lifts
When you want to add weights, but you know you can't push it all the way, get creative with your spotter. Secure a few short, thick boards together. Place them on your chest. As you lift, bring the weights down to the board. This gives you a shorter lift area and helps you get used to heavier weights.