For information on how to use reclass directly, call reclass --help and study the output, or have a look at its manual page.
The three options, --inventory-base-uri, --nodes-uri, and --classes-uri together specify the location of the inventory. If the base URI is specified, then it is prepended to the other two URIs, unless they are absolute URIs. If these two URIs are not specified, they default to nodes and classes. Therefore, if your inventory is in /etc/reclass/nodes and /etc/reclass/classes, all you need to specify is the base URI as /etc/reclass — which is actually the default (specified in reclass/defaults.py).
If you’ve installed reclass from source as per the installation instructions, try to run it from the source directory like this:
$ reclass -b examples/ --inventory
$ reclass -b examples/ --node localhost
This will make it use the data from examples/nodes and examples/classes, and you can surely make your own way from here.
On Debian-systems, use the following:
$ reclass -b /usr/share/doc/reclass/examples/ --inventory
$ reclass -b /usr/share/doc/reclass/examples/ --node localhost
More commonly, however, use of reclass will happen indirectly, and through so-called adapters. The job of an adapter is to translate between different invocation paradigms, provide a sane set of default options, and massage the data from reclass into the format expected by the automation tool in use. Please have a look at the respective README files for these adapters, i.e. for Salt, for Ansible, and for Puppet.