Adding an installment loan will probably provide a small boost in score because of the mix in credit. But to see your scores rise significantly it is going to just take time. The reason the CSR said to be added as an authorized user to someone else with a high credit score is that the account(s) you would be added to would most likely have a long clean history. As you simply tend to the accounts you have and keep reported balances in check your credit score will grow on it's own and you won't have the confusion of someone looking at your report and trying to decide what part of the score is yours and what is attributable to the AU accounts.
Keep your overall reported utilization under 9%. Try to let only one revolving account report a balance each month, but certainly keep it under half of total accounts, and pay on time every time. In a another year you will be on these forums telling everyone "This is how I got such a fantastic credit score".