You need a .p12/.pfx file (contains both your certificate and private key). If you only have .crt and .key, combine them with:
openssl req -new -x509 -days 3650 -newkey rsa:4096 -keyout yourcompany.key -out yourcompany.crt
openssl pkcs12 -export -out your_cert.pfx -inkey smime.key -in smime.crt