If you accidentally deleted a dataset using the Delete button in Organize Datasets you can setup the dataset again by using Create a New Dataset.

If you have permission to create a database, enter the database name to recover as the dataset name.  For example, if a dataset named New Data was deleted, the database name NewData will still exist as long as the database administrator has not deleted the database from SQL.  Select File | Dataset | New Dataset and enter the name of the database you want to recover.  In this example we would enter "NewData". 

Click OK and the following window will appear:

If you select Yes, the dataset will get added to DMS and the system will ask you to login to the NewData database.  After you have recovered the dataset, you can always use Organize Datasets to change the dataset name and/or color.

If you do not have permission to create a database, all you need to do is select new database, enter the dataset name, and then enter the database you want to add to the dataset.