Can't get the plugin to run after installing with Packer

I feel like I must be missing something simple, but for the life of me I still can't get it worked out.

I've installed it by adding this to my `plugins.lua` file:

```lua
use({ "iamcco/markdown-preview.nvim", run = "cd app && npm install",
setup = function() vim.g.mkdp_filetypes = { "markdown" } end, ft = { "markdown" }, })
```

But then I open a markdown file and run `:MarkdownPreview` but the command isn't found. Even before doing any other configuration, should that at least work? Is there a way to check whether it's been installed correctly?