From the developer's web site:
What are the differences between the Lite and Pro versions?
In a nutshell, the Lite version supports read only access to SharePoint servers. The Pro version, on the other hand, allows to add and modify the contents of a SharePoint server. In the case of documents this is achieved using the ”Open In“ file sharing protocol between apps. SharePlus can send office documents to apps such as Documents To Go, and receive the modified version of the documents to update it in a document library.
Also the Pro version provides offline support, which means the ability to synchronize the contents of lists, to take them anywhere, even modify them offline, and then automatically have them synchronized back when the device is again in a connected scenario.
So it would indeed appear that you need the Pro version. Contact the developer to confirm.