Found this useful guide today since I needed to upgrade the memory of an Inspiron 1520. Works with a Vostro 1500 as well. The guide has detailed instructions with pics of how to reach the second DIMM socket, located under the keyboard.
PC Tip Guys - How To Upgrade Dell Vostro 1500 and Inspiron 1520 Ram
PC Tip Guys - How To Upgrade Dell Vostro 1500 and Inspiron 1520 Ram