Presumably, that field holds just numbers. In that case:
1. select the mergefield
2. press Shift-F9 to expose the field code, which should look like { MERGEFIELD address1 }
3. add a numeric picture switch to the field, thus - { MERGEFIELD address1 \# 0;; }
4. press F9 to update the field
5. Run your mailmerge.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]