Make sure you save in the bios setup. It could be the new ram is not compatible with your computer. You got the same speed? Try the same brand if it doesn't work.
Also, try putting it in a different slot. If it doesn't work just take it back and try a different one.
Make sure it's seated in the clips right and make sure you can see it in the bios. Dell Inspiron 1545 should accept ddr2 800 memory. Seeing how that is what the specs for that laptop call for ion memory. It should take up to 8 gigs of memory.