There are three basic methods for adding keyword metadata to photos in Photo Mechanic. You can type keywords directly into the keywords field in the Metadata (IPTC) Info or Metadata (IPTC) Template. You can use the Keywords Panel and Edit Keywords dialog box to import, export, and edit a flat list of keywords for controlled vocabulary. The most advanced option is the Structured Keywords panel, which allows you to import, export, and edit a hierarchical controlled vocabulary. Here are three quick examples of adding keywords to the same photo. 

Entering keywords manually using the Metadata (IPTC)Info dialog box:

Type comma-separated keywords in the Keywords field. Advantage: This is fast and easy. However, you may use inconsistent variations as you add metadata, and that could cause confusion when you later try to sort and find photos based on keywords. 

Keyword lists using the IPTC Keyword panel:

You can find the IPTC Keywords panel by selecting Edit Keywords from the triangle button next to the keywords field. From here, you can create a master keywords list by typing in new keywords or import a plain text file (one keyword per line), and you can export your existing file to share with others. You can also take advantage of snapshots to manage vocabulary for different occasions.

Add keywords from your list by using the dropdown menu next to the Keywords field. You can also combine this with the first method by adding keywords you've entered into your current keywords list.

Using vocabulary lists gives you more control over your keywords and can be set up quickly. However, large master keyword lists can get cumbersome to navigate in a flat structure. For the final option,  structured keywords, select Structured Keywords... from the dropdown menu next to the Keywords field. 

Structured Keywords:

Structured keywords allow you to maintain a hierarchical controlled vocabulary that you can import, export, and edit. You can add entire paths of keywords to each image at once, making your photo metadata more consistent and easier to search. In this example, selecting the Add path button added: "chicken, animals, animal, beast, beasts, creature, creatures, zoology, livestock, poultry" without additional typing. Structured keyword lists require more time to create, however you can take advantage of free and commercially available vocabularies.