This will work for any language you have a syntax file for, but I'll use R as the example.
1. Open an R file, and on the plugins menu, click "Word Complete"
Type a couple of letters e.g. rn and then press ctrl+space (default word suggest keyboard combination) and hopefully an auto-suggest popup menu will appear.
3. If nothing seems to happen, be sure you have configured R with the R syntax file (see previous post), otherwise you may need to do some further configuration.
3.1 Go to Tools > Plugins > Customize Plugins > Word Complete (be sure it has a check) > R and double check the various settings, also verify the keyboard short cut assigned under the keyboard tab.
This post was inspired by Tal Galili's discussion of wordpress themes, and Yihui Xie's Notepad++ autocomplete efforts.