When UAC is enabled windows 8 uses "Run as Administrator" to install software. When double clicking on an installer this then checks the "Administrator" account for the network drives (which the administrator doesn't have mapped) and this is why the drives are not listed.
There are a couple of workarounds:
Disable UAC and this will install the software as the user using their permissions and mapped drives.
Use an elevated prompt to map the drive and then to run the installer.