The selections are designed to find those who have a powerful stroke, the ability to adapt blade angles at high rates and execute a variety of race plans.
It is hard to practice technique without being in a boat. Paddling pool or dockside can help and TRRA has indoor rowing tanks that work so so for paddling. Your best bet would probably be combining pool time with lots of video analysis of top level paddlers (look to the Canadians for a stroke similar to what we will use). A broomstick and mirror are great tools. This is a decent video:
http://www.youtube.com/watch?v=X4DQ2vTT3O8Weight is factored into the erg score, 10 lbs makes roughly 2 seconds difference. Additionally it is not uncommon for people to drop 15 seconds on their erg time just from gaining comfort on the machine and improving their fitness levels. Both of these are achievable in 3 months.
You have 3 months and 2 test dates before the last time trial on May 6th. This is a sufficient amount of time to develop comfort on the paddling erg even if you have never been on it. There are 2 easy ways to get paid access to a paddling erg - buy one ($1,000+) or use the TRRA ergs ($22 per month). You can get free access to an erg by getting a friend to buy one, also after the time trials any spare time can be used for a quick workout.
Training on the paddling erg is the most effective exercise for decreasing your erg time. Use a mix of hard intervals (30%), fast sprints (10%) and light endurance work (60%). Use light endurance work in place of interval and sprint rest periods to decrease your total workout time and improve muscle recovery. (intervals like one minute on/ one minute off, sprint work like 10 seconds all out, endurance work like 20 minutes easy)
If you are not currently doing this type of exercise start the first week with all light endurance work with a few sprints for fun (minimum of 100 minutes per week). The second week add an interval set of no more than 10min on 2 of the days. You can build volume and intensity slowly from week to week. Don't do more than one time trial per week and keep the resistance below 4.
If you dont have access to a paddling erg do cardio work on a machine or running. Strength exercises should focus on those that mimic the stroke (hip flexors, abs, back, & pulling. Pushups and pullups will go a long way, do sets that focus on speed and intensity with lots of rest rather than volume. Core strength and stretching are the foundation to general strength.