hello,
How can i auto populate account custom address fields to contact address fields on selecting account from contact edit view.
As it happens I was working on something very similar today 
You use JavaScript to Override the onclick event on the account selection button.
If you inspect the button element it contains:
open_popup( "Accounts", 600, 400, "", true, false, {"call_back_function":"set_return","form_name":"EditView","field_to_name_array":{"id":"account_id","name":"account_name","billing_address_street":"primary_address_street","billing_address_city":"primary_address_city","billing_address_state":"primary_address_state","billing_address_postalcode":"primary_address_postalcode","billing_address_country":"primary_address_country","phone_office":"phone_work"}}, "single", true );
You need to add your custom fields to the field_to_name_array example: “field_from_account”:“field_to_populate”
In order to override the onclick event add a custom js file like here : http://sugarmods.co.uk/adding-custom-javascript-to-a-view-in-sugarcrm
Then in the the file do something like this to override the button :
document.getElementById("btn_account_name").setAttribute('onclick','open_popup( "Accounts", 600, 400, "", true, false, {"call_back_function":"set_return","form_name":"EditView","field_to_name_array":{"id":"billing_account_id","name":"billing_account","billing_address_street":"billing_address_street","billing_address_city":"billing_address_city","billing_address_state":"billing_address_state","billing_address_postalcode":"billing_address_postalcode","billing_address_country":"billing_address_country","sage_account_ref_c":"sage_ref_billing"}}, "single", true );');
Make sense?