Dear Bahar
You have to specify the priors in terms of both mean and variance. So for example if you expect the source to be at location 10,30,50 then this is the prior mean.
If you don't expect it to be exactly there but within a gaussian centred at this location with standard deviation 10mm, (and you are equally uncertain in all 3 directions) then the prior location variance (10^2) will be [100,100,100] .
The same thing applies to the moments.
Hope this helps
Best
Gareth