I have a MacBook Air, and upgraded to El Capitan OS X a few days ago.
My camera is a Canon EOS Rebel T3 and I'm using the USB cable it came with.
When I plug the camera into the USB port and turn the camera on, Picasa does not launch automatically like it used to. (Picasa preferences read "When a camera is detected, launch Picasa".)
I did this:
Open Finder and navigigate to your "Utilities" folder then launch the "System Information" application.)
e.g.: "Applications" -> "Utilties" -> "System Information"
And yes, my camera shows up when I do this.
The native app, Photos, automatically launches, and I can view the photos with that. I "exported" all the photos to a folder I knew I could find, and then imported them into Picasa, so I ultimately got what I wanted. But I'd like to figure out how to get Picasa to "see" my camera again, and I appreciate any help getting me there!
Use Apple "Image Capture" to assign the application that you'd like to launch.
To do this:
1) Connect the camera to your computer via the USB cable and power it on.
2) If any other application (e.g. Apple "Photos" application, etc.) launches, just close that program.
3) Launch the Apple "Image Capture" application (it's located in the "Applications" folder.)
4) The left-side panel of Image Capture has a "Devices" list, which will show your camera in that list. Click on your camera's name to highlight it.
And here's the tricky bit...
5) After you highlight the device name, you *might* see a box in the lower-left corner of the "Image Caputure" application window which has the camera's name, followed by a selection that reads "Connecting this camera opens:" (and a pull-down menu specifying the name of the application.)
If you DO NOT see that panel, you'll see a very tiny icon in the extreme lower-left corner of "Image Capture" that looks like a box with triangle pointing up. Click that icon and it will open the device connection options.
With the device connection options open, you'll now see the selection list indicating which application (if any) should launch when the device is connected. Apple's own applications will be in the list, but Google Picassa will not be... no big deal, click the line that says "Other..." and this opens a file-selection dialog. Navigate to the Applications folder, find Picassa, and select that icon.
6) Close "Image Capture" (there is no "save" button... the change is made when you change the selection in the list.)
Disconnect and re-connect your camera. Your Mac will launch Picassa.
I have mine set to launch Adobe Lightroom (which also isn't in Apple's default list of photo applications) and now when I connect my camera, Lightroom launches, notices the device, and automatically offers to import my images.
Incidentally, you can also set this for memory cards if you use a card-reader to import rather than a USB cable to your camera.
Here's a image with the window.