[SalesForce] How to display a variable as checkbox in visual force page

When this happens to me it is usually one of two things:

(1) I don't have an component on my page

(2) I am using a rerender attribute on my or that is submitting the form and I haven't added the id of my component, which means that the error is being swallowed and the page just looks like its refreshing - I've explained this point in more detail at:

http://bobbuzzard.blogspot.co.uk/2012/08/the-importance-of-page-messages.html

However, looking at your code I don't think any pagemessages will be added - this section:

     Competitor__c comp=new Competitor__c();
      //List<Competitor__c> checklist=new List<Competitor__c>();
      //checklist=[select Competitor_Machine__C,Competitor_Machine_Description__c from Competitor__c];

      if (comp.Competitor_Machine__C == 'other' && comp.Competitor_Machine_Description__c == null)
      {
              ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.ERROR,'If other is selected please enter a description');//Same old drama 
ApexPages.addMessage(myMsg);
          // Return null so the page won't redirect
        return null;
      }

instantiates a new, empty Competitor__c() record, so none of the fields will be populated, which means you won't add a message. Your code then tries to insert/update the record:

try{
      if(ApexPages.currentPage().getParameters().get('Name')!=null)
      {

      insert cc;
      }
      else 
      update cc;
      }
      catch(Exception e){}        
      return new PageReference('/'+ApexPages.currentPage().getParameters().get('retURL').substring(1,16));
    }

However, if an exception occurs you can't redirect the user to a new pagereference as you are using a rerender attribute on the commandbutton, so I'd expect the page to simply refresh if an exception is thrown.

If you have a list of list "List < List < String>>" you will need to do something like :

<apex:repeat value="{!alist}" var="l">
    <apex:dataList value="{!l}" var="c">
          {!c}
    </apex:dataList>
</apex:repeat>