Apache Lucene.Net uses git as its source code management system.
The official repository is here: https://git-wip-us.apache.org/repos/asf?p=lucenenet.git.
You can clone the repo with the command line or your favorite Git client, example:
git clone https://git-wip-us.apache.org/repos/asf/lucenenet.git
There also is a mirror at GitHub https://github.com/apache/lucene.net which is synced from the Apache repository.
Most work currently happens on the branch named branch_4x.
The guide for building/testing is currently in the Git repository here