Android - Unable to Install Certificate

Just had the same issue.

Go to Settings > Security > Install from storage.
Locate your certificate file and install it.

If instead of Install from storage preference you have Install from SD card (this is what being displayed when you use API-19 emulator for example), then one extra step required. Because in this case the activity shows only Recent and Downloads menus without simple way to pick the certificate from the sdcard.
The workaround for me was to put the certificate on my desktop's local web server (mongoose for example). Then (on emulator) I used the stock browser to open the web directory, tapped on certificate (see below) and selected Save link. The file will be downloaded and shown in Downloads view and you will be able to pick it successfully.

save link screenshot

Another caveat: when installing a certificate via Settings > Security > Install from storage on my Nexus 7, Android 5.0.2 I had to navigate to the file via Internal Storage:

Navigate via Internal Storage

Trying to select the file via the shortcut to Downloads did not work.