Click this text field then go to the last field name (currently its 'Has_Division_Values__c') then add a comma ( , )

OR(
 State_of_the_Port__c ,
 Holiday_Party__c ,
 Connections__c ,
 Nuts_Bolts__c ,
 FlySEA__c ,
 Evergreen__c ,
 Equity_in_Action__c ,
 Pier_to_Pier__c ,
 Air_Mail__c ,
 Office_of_the_Commission__c ,
 Commissioner_Calkins__c ,
 Commissioner_Cho__c ,
 Commissioner_Felleman__c ,
 Commissioner_Hasegawa__c ,
 Commissioner_Mohamed__c ,
 Century_Agenda__c ,
 All_Press_Releases__c ,
 Corporate_Press_Releases__c ,
 Maritime_Press_Releases__c ,
 SEA_Airport_Press_Releases__c ,
 Environmental_Program_Updates__c ,
 Green_Corridor__c ,
 Sustainable_Airport_Master_Plan__c ,
 NEPA_SEPA_Notifications__c ,
 Environmental_Excellence_Awards__c ,
 Community_Events_and_Programs__c ,
 Diversity_in_Contracting__c ,
 Highline_Forum__c ,
 STart_Working_Group__c ,
 South_King_County_Community_Impact_Fund__c ,
 Tourism_Grants__c ,
 Bell_Harbor_Marina__c ,
 Fishermen_s_Terminal__c ,
 Harbor_Island_Marina__c ,
 Salmon_Bay_Marina__c ,
 Shilshole_Bay_Marina__c ,
 Miscellaneous__c ,
 Has_Department_Values__c ,
 Has_Division_Values__c 
)

Without clicking anything after last action above, Click 'Insert Field' button.\
(Make sure that the cursor is still after the comma you just added)

Choose the CHECKBOX field you want to add. 

For now, let's go with "Dummy Field (for SFMC)"

Is Marketing Eligible: Adding A New Checkbox Field to the Formula