Others have described the reservation system, and at this point in the year, trying to secure a reservation in one of the major tents, the two of the busiest nights of the fest (opening and closing nights and weekends are the busiest) is not likely.
Your travel agent going through a Concierge probably won't work and will be expensive if it did. I woud also question whether either of them know any more than you about securing a seat, if they did not immediately explain how the system works.
Your best bet, might be to a bit of research yourself, and wind up going with a ticket reseller. People will buy blocks of tables, then sell "tours" that take you around for a bit then take you to their table for a while. Just verify that the place is recognized business, and how much time in the tents you get, what is included (beer? Food?)
Otherwise, as others said you could just go in the afternoon and try to find a seat. It works most days during the week, tougher on weekends, rarely on a weekend night for a group of four.
Edit: Should add, Yes "scalping" reservations is not allowed and buying tickets privately, or on Ebay is not advised, however if time in a tent is included in a "tour", then that is allowed. The tour part ups the cost, sometimes it is linked with hotel rooms or just a walking tour, they may not include beer and food (the people who secure the reservation can strip off the food vouchers and sell separately), and as I implied above, they may limit time, with the plan of seating multiple groups in an evening.