Well Vdd, which is Vbatt, is 3V, and this is within the voltage range of 2 AA NimH cells. Solar panel will need to have a voltage output of around 3V to charge it, so you will probably be using a 6V panel, and relying on the battery to absorb excess charge. As to the LED you want 2 in series, and size the inductor to have around 100mA of current through the LED's, as the main reason to go to a series connection is driving multiple LED's, so a 2S arrangement of the LED's is used, and you need a higher power output in the same package. 3S might work, even though it means the inductor pin will see around 9V peak voltage, which is a little above the 5V rating of the part, and some might fail long term.