If you are on an Android device please remember to active the Media Device option on your phone or tablet. Go to Setting > Storage > Options (upper right corner) > USB computer connection > and select the Media device (MTP) option. When you connect your Android device via USB to your PC, you should be able to see all the folders on your device. 

The new Media Transfer Protocol (MTP) enables you to manage content on any portable device with storage. MTP was first shipped as part of the Windows Media Device Manager 10 SDK. This porting kit includes the complete MTP specification, updated design documentation, updated sample source code, software to implement a PC-based MTP simulator, and MTP test tools.

If you are looking to connect your Android device to Mac computer, please check out Android File Transfer for Mac.