Hi,
Which plugin are you using? You can check this from "Server -> Console" page. Check Plugin Type. If it is "Native" you need to make sure to configure Internet Explorer to allow Activex Plugin installation. (Add iDRAC IP/Hostname to Trusted Sites and Security level as Medium). If you are using Java plugin make sure JRE 6 or above is installed on the client. More information available on iDRAC7 user Guide (Link mentioned below). Refer section "Configuring Web Browsers to Use Virtual Console"
ftp.dell.com/.../integrated-dell-remote-access-cntrllr-7-v1.30.30_User%27s%20Guide_en-us.pdf