The drop-down list for Output:Variable and Output:Meter are special and actually take the list of variables (or meters) from the RDD and MDD files. Since these files are only generated when the file is simulated, to fix this problem, run the simulation for the specific model you are editing. It will need to include the "Output:VariableDictionary" input object with the key field set to "Regular" in your file too:

Output:VariableDictionary,Regular;

After you simulate the file, you should have an RDD and MDD file, and that drop-down list should be populated. You might need to close and reopen the file in IDF Editor for it to read the RDD and MDD files.