VMD can read in new coordinate sets from either PDB files (to add a single new frame to the animation), from ascii CRD files (which may contain several frames), or from binary DCD files (which may also contain several frames). The new coordinate sets are appended to the end of the stored animation list for the selected molecule. At present, there is no way to use Babel to append non-PDB and non-CRD/DCD files. The format is determined by selecting the appropriate name in the File Type chooser. If you want to read in all the information from the file, make sure that the Amount chooser says `All', then press the Read button in the bottom right corner. This will bring up the file browser so you can select the file. (Unlike the Files form, the file is loaded immediately after the filename is entered in the file browser.)
Sometimes you may not want to read in a whole CRD/DCD file. For example, you may only want the last frame, or every tenth frame. You can do this by choosing the `Selected' option in the Amount chooser. This brings up the frame skip selection controls. Once you've chosen the appropriate values, press the Read button to bring up the file browser and finish as mentioned in the previous paragraph. The Amount options are ignored when reading in PDB files.