Totally stuck on validation rule, making a lookup field mandatory....

Hi Guys,

I have been stuck for over 2 weeks trying to create a validation rule so that if any of these pick list values are selected

ISPICKVAL(Visit_Type__c,"New Prospect"),

ISPICKVAL(Visit_Type__c,"Existing Prospect"),

ISPICKVAL(Visit_Type__c,"Trading Client"),

ISPICKVAL(Visit_Type__c,"Client Upsell"),

ISPICKVAL(Visit_Type__c,"Conference Call"))

A look up field called “what” (which is a standard field) the opportunity and the companies(accounts) part of the lookup field  becomes mandatory,

I have had some feedback on a previous question saying that I should use the

objects Unique 3 digits identifier which for Company is 001 and for opportunity it is 006,

the problem is all the validation rules I have wrote have not worked, could anyone give me some advice please this would be greatly appreciated

Thanks

James