By default, ESXi only allows signed vSphere Installation Bundles (VIBs), because an unsigned VIB represents untested code installed on an ESXi host. You can change the acceptance level for each host, in the Configure | System | Security Profile menu, under the Host Image Profile Acceptance Level option:

The host image profile supports the four acceptance levels:

• VMware Certified: VIBs created, tested, and signed by VMware.
• VMware Accepted: VIBs created by a VMware partner but tested and signed by VMware.
• Partner Supported: VIBs created, tested, and signed by a certified VMware partner.
• Community Supported: VIBs that have not been tried by VMware or a VMware partner. Community Supported VIBs are not upheld and don't have a computerized mark. To ensure the security and respectability of your ESXi, don't permit unsigned Community Supported VIBs to be introduced on your hosts.