Mounting a volume means assigning an alphabetical identifier to it.
You need to mount the VHD volume in order to browse its files and folders. See Exploring VHD Volumes.
Follow the instructions below to mount a VHD volume.
1. Launch the R1Soft Hyper-V VHD Explorer.
2. Select an attached VHD in the "Attached VHDs" pane.
3. In the right pane, right-click a Volume contained in a mounted VHD. Select the "Mount" menu option from the appeared menu.
In the appeared pop-up window, you can select one of the following options:
4.1 If you have chosen the first option, then select a drive letter from the drop-down menu.
4.2 If you have chosen the second option, then click "Browse..."
In the appeared window browse your PC and select an existing empty folder.
Using the "Make New Folder" button create a new folder in the desired place.
Then click "OK" to close the "Browse for Folder" window.
The path to the selected folder will appear in the folder field.
5. Once the option is selected and the drive letter/folder is specified, click "OK."
6. Then that drive letter appears in front of the appropriate volume in the "Drive/Path" column in the right pane.