The first time you log in, Timmr will automatically load it's dashboard with Saleforce leads, contacts and accounts and then begin analyzing them for duplicates. Once the analysis is completed, the Trimmr dashboard will display a Dataset for each object (along with the total number of records and duplicates in each). Datasets will also be shows in the left navigation menu.
New datasets can be created or existing ones edited. Please refer to this article: Deduplicate Custom Datasets.
To work with a specific dataset, select it on the dashboard or from the Navigation menu.