The MP3s are easy - to do these in bulk, use a 3rd party ID3 tag program such as mp3tag. This will allow you to make bulk changes (such as set the Album/Year/Artist atributes for all files in a folder, useful when doing an album), it also allows you to set tags based on a matching pattern using the file name, and to query online databases.
Once the ID3 tag is set inside the MP3 file, LMCE's updatemedia scanner will read these tags into the LMCE library database and you are done. Unfortunately, doing the same for DVDs isn't possible, as there is no embedded ID3 tag, the data is stored in a .id3 file next to the main file, so you will probably need to use the web admin/online tools. Worst case you can set any attribute manually for any media file in the web admin by going to the Media File Sync option menu, then navigating to the file using the left and right panes, once displaying the files properties in the right pane, you can add/delete/modify any attribute from there.