Newer Proteus versions include a built-in Library Manager (Tools > Library Manager). However, the WS2812 is not in the official repository, so you must still use manual installation.
If Proteus is running, save your work and exit. The library files are locked while the software is open. ws2812 proteus library download install
Here’s a short, practical guide to get WS2812 models/libraries into Proteus and use them with Arduino code. Newer Proteus versions include a built-in Library Manager