Answer
The option to look for Microstation design files with any file extension may be unchecked under ArcCatalog’s Tools > Options dialog box. You can check this option on the CAD tab to make ArcCatalog examine all files in a folder to see if the data is in Microstation design file format. Also, make sure the 'CAD feature datasets, feature classes and Drawings' is checked in the General tab’s 'Which types of data you want the Catalog to show?' list box.