Model Menu
The model menu lists every asset pack. You can place models from all of them, including packs you have not unlocked, and when you are not signed in.
The model menu is visible on the right side of the screen. It will be replaced by more specific options when you have certain tools and casters active. So if you can't find it, press V on your keyboard or click the topmost button in the toolbar to switch to the select tool.
Now the right side of the screen should look something like this:
The model menu inside the right menu.
Type Buttons
First, you have the type buttons on the left side of it. These decide what type of models you can see in the menu.
You can switch the content of the menu by pressing another type button. The active type is indicated by a grey background and an orange bar on the side of the button. The topmost button is the hierarchy button and toggles the hierarchy menu. You can read more about that menu here.
Filter Menu
Secondly, there is the filter menu. You can search for models by typing their name or other information about them into the search bar. You can expand the advanced filters, by pressing on the 'Filters' button. Here you can filter by theme or form and even category if your active type is 'Attachables'.
Filtering models by theme or form.
You can have multiple themes or forms selected to show models of either one in the menu, or select no filters at all to show every model.
Objects List
And lastly, the models themselves. They are sorted by their set, in alphabetical order, with the latest release being highlighted at the top of the menu. If you click on one or drag them out onto the build plate, the model will load into your scene.
By clicking on any asset pack's name, you can collapse it to make the menu more tidy and concise.
Packs you have not unlocked
Packs that are not unlocked on your account are listed under Not bought. Their header shows this icon: money_off
You can still place those models. The first time you use a pack, Townbuilder tells you that its models can be used to build, but the building cannot be downloaded until you unlock the pack.
Use Unlock on the pack header, or buy the pack in the shop. After it is unlocked, buildings that use it can be downloaded. Guests can place the models. Saving, sharing, and downloading need a signed-in account, and download also needs every pack in the building unlocked. See Export Buildings.