wisherut - which version of Confluence are you trying to install plugin into? Also, if you have upgraded the UPM (Universal Plugin Manager) the user interface is probably different, so if you can confirm the version of that I can be much more helpful.
In terms of manually installing a downloaded plugin, there is usually an upload link that allows you to upload the plugin yourself. The specific location / page this is on depends on the version of UPM.
If you have any further information (such as a support ticket where you've been asking for help) this may give me some more context to get you going much faster.
wisherut - sorry that OnDemand doesn't allow you do install the plugins that you need. There is a ticket for tracking interest in including the Source Editor in OnDemand - see
CONF-24922- feel free to indicate your interest here.There are some technical and security reasons while plugins cannot be installed in OnDemand hence the limitation. There's some more information in the Atlassian OnDemand Plugin Policy.
In summary, if you want to install any plugin in your Confluence instance you will need to host Confluence yourself (or use a third-party server hosting service).