Try to close SPD and clear its cahce before adding. The cache is located here:
%APPDATA%\Microsoft\Web Server Extensions\Cache
UPDATE1: It has become much clear after comments.
The cause of your issue is the fact that you added fields to a list, but not to content type. As your workflow is reusable it only "can see" fields that are in content type it is associated with. Whereas list workflows can see fields which are in the list.
So in your case I can see two solutions:
a) create your own content type derived from Event (MyEvent for instance) and add necessary field there. In such a case you can create reusable workflow associated with this content type and all the fields will be visible.
b) create list workflow. In such a case you will be able to use all the fields which are in the list.
UPDATE2: As you use standard reusable workflow, you can use Solution a). For that first you have to create Content Type derived from Event. You can do it as follows:
After that create some Site column (Select Site Columns in the left panel and press New Column). After creating site column don't forget to press Save button.
Now you should add your site column to new content type:
That is alomost all. Now you have to go Copy/Modify reusable Approval workflow and select your new MyEvent content type for that.
Now you should be able to have access to all the columns in your MyEvent content type!