Do these check boxes have to be the sort you can insert using the Forms
toolbar? Or are they more like ticked and crossed boxes that you want to
appear depending on whether a field in your mail merge data source is Yes or
No (or 1 or 0, etc.)?

If it is the latter, then you can use an IF field where the result is either
a ticked box or a crossed box depending on the value.

e.g.

{ IF "{ MERGEFIELD myfield }" = "Y" "<ticked box character>" "<crossed box
character>" }

where all the {} are the special field code braces that you can insert using
ctrl-F9.

Instead of <ticked box character>, you need to insert a suitable character ,
for example characters 253 and 254 (OxFD and 0xFE) from Wingdings, 81-84
from Wingdings2.

Peter Jamieson