At apple. Try this one
http://www.apple.com/support/downloa...lease4ppc.html "IF" you have a ppc mac. But if you have the new intell mac then use this one
http://www.apple.com/support/downloa...ase4intel.html
If it won't let you reinstall it, go to the Receipts folder & remove the previous last Java update.
Receipts folder sample image
Scroll down to see the Tiger equivalent. Remove only J2SE50Release4.pkg
Remember to do a safe boot & repair permissions after installing.