Let us start from the beggining. Re-install blockland in case of the likely event that you forgeted it up. If you want to save lots of hastle download Return To Blockland (By Ephialtes) then save it in my documents or desktop. Next go Start-MyComputer-HPPavilion-ProgramFiles-Blockland. Then drag the folder return to blockland into the file named add-ons. Open up blockland and you'll see that you have a new section of GUIs to explore. Click on mod manager and find the add-on you want. Click on it then scroll down under the info of the add on and hit the download button. Wait a bit for it to download then enable it and start a server.
Mod manager does not contain every add-on so if you can't find it search on here and install it the way you installed RTB. RTB is located in the add-ons section of this forum.
RTB Archive
www.returntoblockland.com/archive is the link to all the add-ons from the previous version of RTB download them in the usual way but before putting them in your add-ons folder you have to rename them.
If it is a weapon you put a Weapon_ before the name of the weapon.
Map_ for maps.
Script_ for scripts.
Server_ for server sided mods.
There are more but I'm sure you have enough common sence to figure them out for yourselves.
If you're unclear of anything, just ask.