1. Open your DBM GUI. Go to Configuration > Volumes > Data Volumes. Right click on the next data volume (greyed colour) and choose New Volume.
2. Insert the Size, Device/File and Type (easy to refer to the previous data volume). Click OK. You are now successfully inserted a new data volume!
3. For log volume. The steps are the same as per step 1 & 2. Just, ensure to select Log Volumes.
Alternatively, you can also set the data area to be automatically added by referring to this article, How to Automatically Add MaxDB Data Area.