When posting a new topic, please ensure that you select the correct category for your post in the top drop-down box of the edit window. The default entry is the first category shown on the All Categories page; this is unlikely to be the category that you want. The Category drop-down box will be present if you click the New Topic tab in the Forum menu; if you are viewing a particular category of the Forum and you use the New Topic button in the Category Header section, the drop-down box will not be present, and your new post topic will automatically appear in the category that you are viewing.
