There are a few possibilities. First, a very colloquial, conventional sentence: "Ready when you are!" It's not formal, so it may not quite fit your criteria, but it is a very common idiom.
Another, and this is a bit more polite and formal, would be, "I'm ready whenever it's convenient for you." This makes it clear that you're not demanding assistance right now; you're ready, and your senior should choose a time that works best for him.
If your schedule isn't quite that clear, it's best to give a short outline of your availability: "My schedule is open from 10am until noon, and again from 3pm until the end of the day. What time would be best for you?" Again, you're giving your senior the choice of times, though this time within some constraints.