You can either

a. view the ADDRESSBLOCK field code using Alt-F9 and use ctrl-F9 and the
keyboard to insert e.g. {
MERGEFIELD Job_Title } inside the text that says
<<_COMPANY_

so you have

<<{ MERGEFIELD Job_title }
_COMPANY_

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

However, editing ADDRESSBLOCK code is not recommended. If you do it, test
carefully, and ensure that you never edit the Address block properties
because non-standard stuff is always discarded. Or save the block you need
as an Autotext or something so it's easy to re-insert.

b. Dont use ADDRESSBLOCK. Use the "individual fields" approach where each
element
of the address is inserted as an individual { MERGEFIELD } field and { IF }
fields are used
to deal with e.g. missing lines and other "trim".