If you want to get the API (Javadoc)
- this will be resolved by your IDE if you include the library in your project follwing the instructions here http://opensmpp.org/downloads.html
- if you want to download the API manually start to have a look here https://oss.sonatype.org/content/groups/public/org/opensmpp/
For examples you could have a look in the unit tests, clone (or download) the project https://github.com/OpenSmpp/opensmpp.git
and have a look for sources in the 'client/', 'sim/' or '*/test/' folders.
For testing purpose this tutorial might be also of interest: http://mobiforge.com/design-development/sending-sms-with-smpp-kannel-and-java