what replacing proxy basically means if the sim you downloaded has on a pair of jeans that is only avaible in late night and all you have is the base game when you play with that particular sim the latenight jeans will be replaced with a base game jean.
Place the grave in your sims home inventory in buy mode. Then use the the cheat testingcheatsenabled true. Hold shift and click on the lawn that you want to place the tombstone on and a button will pop up saying buy on lawn or something to that extent :s. Then just place the tombstone on the lawn.