In order to make use of the additional information about mature regions on the precursor miRNAs in miRBase, you need to use the integrated tool to download miRBase rather than downloading it from http://www.mirbase.org/:
Toolbox | Transcriptomics Analysis () | Small RNA Analysis () | Download miRBase ()
This will download a sequence list with all the precursor miRNAs including annotations for mature regions. The list can then be selected when annotating the samples.
The downloaded version will always be the latest version (it is downloaded from ftp://mirbase.org/pub/mirbase/CURRENT/miRNA.dat.gz). Information on the version number of miRBase is also available in the History () of the downloaded sequence list, and when using this for annotation, the annotated samples will also include this information in their History ().