I am serving as a missionary for the Church of Jesus Christ of Latter Day Saints in Hong Kong , it is a full time non-pay functions ; do I use my address in HK or US ?

For purposes of filing your US income tax return, you may just as well use your United States mailing address; although technically speaking it really should not matter.  If you are serving overseas (Hong Kong) as an LDS missionary, for no monetary compensation, then you really have no foreign earned income to report on your tax return, and thus no foreign earned income to try to exclude (which someone can otherwise do if they live and work overseas, for a private organization, for more than 330 days).

However, that doesn't appear to apply to you, doing purely missionary work.  So, for the sake of simplicity, if you still receive mail at your US address (and someone collects it there for you), then you can safely use your US address on your tax return.  Otherwise, you can use your (temporary) Hong Kong address.  The main purpose of providing the address information will be so that if the IRS ever needs to contact you, they'll be able to write you a letter, send you a notice, or mail you a tax refund.  If that is more likely at your US address than your Hong Kong address, then chose the US address.

Thank you for asking this question, and good luck with your work.