If BP.tpk isn't working, grab the unsigned_SBP.tpk, it's a modified version of BP.tpk, you sign and install the modified version in case you missed it in the alternative way...
If you got the right tpk, make sure the sign.bat showed no errors, it is possible that sign.bat would produce the tpk but with bad signature and wouldn't install, if there were no errors, did you grab it from install_me folder? If yes, then it might be certificate problems, create a new set of certificates as the ones you used might be bad, probably DUID wasn't caught and stored correctly or something...
I have listed all possible reasons why it might be not working, go through each and hopefully it would work...
Reset the watch and install the BP again, if it still doesn't work, it might be a sensor issues, open the watch dialer and "*#0*#", this will bring up the test mode diagnostic for watch, just like the phone, scroll down to HRM and test all and see if you get readings, if you're not getting any, your watch sensor is defected, reach Samsung for support...