What Tomcat needs is the certificate and its private key. The certificate is public information that any of your user can see, but the private key should be yours only: this is what prevents others from running a website with your certificate. By importing MyCompany.der, you’re only importing the certificate.
You would need to find where you private key is first. (Normally, even the person who issued the certificate to you shouldn’t know its private key.)
The private key may have been generated within your browser during the certificate application process. Try to see if you can export in .p12/.pfx (PKCS#12) format: this should bundle the private key too if it’s there. If so, you should be able to use the resulting file as a keystore directly using the PKCS12 store type: keystoreFile="store.pfx" keystorePass="password" keystoreType="PKCS12" (you probably won’t need a key alias, since there will only be one key entry).