gpu-manager cannot find Nvidia gpu

On Ubuntu 15.04, gpu manager cannot find my gpu. I have a Nvidia gtx 760 and am using the proprietary drivers (346-updates)

output of systemctl status gpu-manager.service:
● gpu-manager.service - Detect the available GPUs and deal with any system changes
   Loaded: loaded (/lib/systemd/system/gpu-manager.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Sat 2015-04-25 10:56:48 CDT; 26min ago
  Process: 670 ExecStart=/usr/bin/gpu-manager --log /var/log/gpu-manager.log (code=exited, status=0/SUCCESS)
 Main PID: 670 (code=exited, status=0/SUCCESS)

Apr 25 10:56:47 knil-PC systemd[1]: Starting Detect the available GPUs and deal with any system changes...
Apr 25 10:56:47 knil-PC gpu-manager[670]: /etc/modprobe.d is not a file
Apr 25 10:56:47 knil-PC gpu-manager[670]: /etc/modprobe.d is not a file
Apr 25 10:56:47 knil-PC gpu-manager[670]: /etc/modprobe.d is not a file
Apr 25 10:56:47 knil-PC gpu-manager[670]: /etc/modprobe.d is not a file
Apr 25 10:56:48 knil-PC gpu-manager[670]: update-alternatives: error: no alternatives for x86_64-linux-gnu_gfxcore_conf
Apr 25 10:56:48 knil-PC systemd[1]: Started Detect the available GPUs and deal with any system changes.

also, I'm not sure if it's connected but this is a line in journctl : Apr 25 10:56:47 knil-PC nvidia-persistenced[485]: Failed to query NVIDIA devices. Please ensure that the NVIDIA device files (/dev/nvidia*) exist, and that user 117 has read and write permissions for those files. user 117 is nvidia-persistenced, the device files do exist, and it should be able to access them.