If you are having trouble accessing your DLC, the first thing you will want to do is check in-game to make sure it is being detected as installed there.
You can do this from the main menu of the game:
1. Select the Manage DLC option from the list of options
2. Select the DLC you are having trouble with and check if it is showing as installed, or if there is an option to purchase it
If it shows as being installed, go into your game and check if you have a starmail for the DLC from entering your Ranch house. If it is installed properly, it should be available; so go find those fashion pods!
3. If it is not showing as installed, find Slime Rancher in your Steam Library, right click the file, and select Properties
4. Select the DLC tab
5. Check the box to install your DLC
If the DLC is not showing here, then contact Steam, as this would indicate that it may not be purchased on this account
6. If the box is already checked, try unchecking it, restart Steam. Now, navigate back the the DLC tab and recheck it to see if that corrects the issue
Please keep in mind that DLC will not work across shared Steam accounts, so double check if that may be the case for you.