There are two ways to upload files using BoxCryptor for Android:
1) Open the BoxCryptor app and login to your encrypted folder. After that you can see the "Upload" button on the top left. After tapping on it you can choose another app in which you can choose the file to upload (e.g. ES File Manager).
2) Open an app which displays your file you want to upload (e.g. ES File Manager or Gallery). Now use the Android "Share" function. After that you can choose the BoxCryptor app as target app. BoxCryptor will open and you can choose where to upload the file.