How to donate to public works projects
How do you get your player to donate money for the public works projects? There is nothing there to collect the money. It's just Lloyd whizzing around in his little square cube.
Each town can feature 30 projects at maximum, and only one project can be under construction at any given time. Lloid is in charge of taking donations for projects, and will stay onsite of the project 24/7 until it is complete. Villagers in the town may donate small amounts each day, but the majority of the funding will have to come from a player or it will take a long time to complete. Once the project is funded, it will be fully constructed the next day at 6 AM. If the Mayor player chooses to participate, Isabelle will host a completion ceremony with up to four villagers present to commemorate completed project.
You should just be able to talk directly to him (Like click like you would talk to any other villager) and donate money towards it, up until its completion. It's not some collection bucket, he himself takes the money.
If you didn't actually choose and set a project you still have to do that first. Talk to Isabelle and then walk with her to pick a place to put the project if it requires. Then Lloid will sit at that location until it is completely funded. (Villagers hardly donate money, so it's up to you).
The quoted text is from the wiki.