ASDM is a Java-based application. You generally need Java 8 (JRE 1.8) installed on your computer. Step-by-Step: How to Download the ASDM-IDM Launcher
If the ASA runs an old ASDM version (e.g., 6.4 or 7.2), you can access it via: https://<asa-ip>:443 (accept the self-signed certificate) Click "Run ASDM" to download a .jnlp file. Launch that with Java Web Start. From there, many versions allow you to install the standalone launcher via the Tools menu. cisco asa 5505 asdm-idm launcher download
On the Cisco ASDM landing page, click the button labeled "Install ASDM Launcher" . ASDM is a Java-based application
If you see a certificate warning, it is safe to proceed (the ASA uses a self-signed certificate by default). 6.4 or 7.2)