A few thoughts off the top of my head:
1. If you can, offer a discounted promotional rate for tasks on your site, where some tasks will be completed for free. Tasks must be approved (by you). Then, just do the freelance work yourself at below-market prices.
2. Post tasks yourself. When people complete the training, pay them to help develop your app / site.
3. Offer discounted rates where you subsidize the income of the freelancer.
4. Offer badges for completing training, and then auto-create a freelancing profile for the user to advertise on other competing freelance site. Yes you're driving away your freelancers initially, but you must offer them something of value or they have no incentive to participate.
The problem you're having is not retention, per se. You are trying to be a middleman and create a new market. The problem with creating a market is that you need to ensure BOTH sides of the market are up and running (buyers and sellers) before you start to attract organic participants. This is the same problem Uber faced and ultimately conquered. Creating a market is hard, and almost impossible to do without large amounts of funding.