How can I download vCenter Server 5.5 (free download)? I tried to download but I only could get VMWware Software Manager. And I couldn't download any further.
All the downloads for vCenter available on the VMWare website. You can download all the available packages from that page. You can also select the versions you want to download the packages.
Support page: https://my.vmware.com/web/vmware/details?downloadGroup=VC55U2&productId=353
On a side note, the VCenter Server is not a free product. It needs purchasing: https://www.vmware.com/products/vcenter-server