The adapter says 3.7V, 300mA. With that sort of current, you shouldn't leave the batteries connected very long at a time. The voltage on the batteries will go up quickly, but the actual amount of charge will not. Because of this, you have to disconnect the batteries frequently to let them "cool down" (though they usually don't even get warm, the voltage will quickly drop down to around 1.5 V after disconnecting). After they have leveled off (usually around 1.5V) put it back on the adapter for another minute or so. Repeat this a bunch of times to get the charge up. I've just started experimenting, but I was able to get enough charge in two completely dead batteries to start up my power-hungry Canon digital camera and take a few pictures (with screen and flash) and turn it off without it dying on me (died the next time I turned it on). With a better circuit, you could automate the on/off cycles to charge them up.
Step 1: Get your power source!
You also don't want really high amperage. Since high current charging leads to leaking batteries, try to find as low current plug as possible (look at the mA rating). A good charger has only 65 mA or so going through the battery, but mine has 300 so that's why I have to keep cycling the power to the battery.