If you want to add a skill from a marketplace , you need to add the marketplace first.
/plugin marketplaces add claude-plugins-official
Once the marketplace is added, browse and install a plugin from the Discover tab.
It will ask you a question about the scope of the plugin. Select the one appropriate for your use.

After this run /reload-plugins to apply.
/reload-plugins
Reloaded: 1 plugin · 0 skills · 6 agents · 0 hooks · 0 plugin MCP servers · 0 plugin LSP servers
To verify the plugin is installed, run /plugin list.
/plugin list
Installed plugins:
frontend-design@claude-plugins-official (user) ✔ enabled — run /reload-plugins to apply
Check the installed skills with the /skills command.

How do you use the skills?
Skills can be triggered either automatically, by phrases in your prompt, or manually. For example, the frontend-design skill triggers automatically if your prompt includes phrases like “Design a settings panel with dark mode” or “Create a dashboard for a music streaming app”.
For manually triggering the skills: you can run
/{plugin}:{skill}