Is this what you're looking for?

SELECT a.COMCODE, a.ACTCODE, a.ACTDESC

FROM ACINF a

INNER JOIN ACPMNTA b ON b.ACTCODE = a.ACTCODE

WHERE a.COMCOD = 3305

AND a.ACTDESC = '';

I'm joining on ACTCODE because I read that you wanted to find the rows in ACINF where ACTCODE is present in ACPMNTA. I included the WHERE clause from your middle query, but you don't have to if it isn't needed.