If you want to create a new Report addon, you wont need any new dev environment. Everything you need is already set up.
The following will create a new folder for your report (change ReportName as appropriate) in the default location for addons for gramps51. You can create your addon in this folder and test it.
cd ~/Library/Application Support/gramps/gramps51/plugins
mkdir <ReportName>
cd <ReportName>
The description on how to write a new report is at Report-writing tutorial - Gramps
You might also look at a SampleReport as an example.