Online Package Manager is available for Lazarus 1.8.0 and above. It seems you're using Lazarus version below 1.8.0. It's time to upgrade.
But if you want to try manual install a package:
Lazarus main menu > Package > Open Package files (.lpk) > browse to the location and select the package (.lpk file) > Compile > wait some seconds, then Use > Install > you will be asked to rebuild the IDE, answer yes and wait some minutes then your IDE will be closed.
Note:
SourceForge currently is down for download.