I can not comment on the system, as I do not have enough reputation yet, but I hope the system admins will allow me to put my answer/comment here.
You mentioned popup form, is that a requirement? If you need a popup form, then it may be a visualforce wizard/form which you may need to create using Apex, or another way is to create custom buttons and use some URL hacking to create new records.
Below is a link to something I did, using a new button to create new records on custom objects - see if that might give you some ideas :
Creating a stopwatch in salesforce which calculates total time elapsed without using apex or visualforce
When you create a new button, Salesforce will ask you how the page will act when you click on the button. There will be an option for popup. In my example, I chose to keep it on the same page.
If a popup form is not a requirement, then once you create a new custom object, make it available to view for the users by create a tab for it. Once a tab is created, users could click on it, and then there will be a button for new record there. However, at this point, you must be wary of permissions - roles and profiles and any custom Public Groups or permission sets which might restrict users from viewing the object and the records.
I know this is a very generalised answer, but hopefully that is a start.