How do I put on weight? I'm a lean man with a busy job and little time to work out.

Firstly, you should calculate your Body Mass Index to ascertain if you are classified as underweight. With this information in hand, you'll have a better idea of how much weight you need to gain. Muscle is heavier than fat and I assume that's what you'd like to grow. Suggestions:

Ensure you stick to a structured fitness program that is tailored to your needs, and designed by your fitness instructor. If you need extra encouragement consider enlisting a personal trainer to support you in getting started.
if you work physically most days, remember that your nutritional intake must meet your energy expenditure (a good portion of protein with each meal, plentiful carbohydrates, and graze often to maintain energy levels).
include healthy fats in your diet from fish, avocado, olives, nuts etc. Healtheries Omega Advanced range will provide you with high quality supplements to choose from if your diet is lacking.
purchase a mega weight gainer protein powder, read the label carefully and commit to taking at least 2-3 servings a day (in between meals and post-workout). Add plenty of fruit and yoghurt to your protein shake for increased bulking up potential
vitamins and minerals will assist increased energy. Essential items to include in your daily regimen are vitamin B complex, magnesium, and an amino-acid supplement.

Decide what your ultimate goal is (for example it might be 80kgs), place pictures up around the house, set the vision in your mind daily and reward yourself for getting closer to your achievement. I'm sure you'll get there if you maintain discipline because each step you gain will enthuse you to go that little bit further.