Try something like:

=IF(ISNA(MATCH(1;INDEX((Activities!$A$1:$A$100=[Address Code])*(Activities!$B$1:$B$100=[Company]);0);0));"";INDEX(Activities!$C$1:$C$100;INDEX((Activities!$A$1:$A$100=[Address Code])*(Activities!$B$1:$B$100=[Company]);0);0))
where Activities B1:B100 contain the Company names to match up with reference on sheet 1 [Company]

Also note: You can't use whole column references with this formula.