I will attempt to explain the ladder making process the best way I can:
Initially construct your ladder:
Next texture your ladder to your satisfaction and then create a brush like the next picture and texture it with comon/ladder, overlap the ladder itself by a fair margin in width,height and depth as the next pic shows, then select your new brush (with the comon/ladder texture) and right click and select "function ladder"
now you should press "N" button - the entity button which will bring up a new menu offering possible angles for this ladder, now this is vital as to whether your ladder works or not.
In your top screen of radiant after your ladder brush is selected and you have pressed "N" you should see a set of possible angles you can select and hopefuly the next picture should help you select the correct value:
Hopefuly this will point you in the right direction and good luck in the future.