First off--if you've installed a plugin, and turned it on, you should be able to access it through the FreeNAS web GUI. In the left column, expand Plugins and click on the plugin in question--it should pop up with a box that has a link to the actual plugin. Failing that...
I'm going to get really basic here--I apologize if it sounds like I'm being insulting (that isn't my intent), but it really is sounding like there's something fundamental that is being missed.
Your FreeNAS server has an IP address. That address is usually expressed as xxx.xxx.xxx.xxx, where each xxx is a number between 0 and 255. Usually, for home networks, the IP address is going to be somewhere in the range of 192.168.xxx.xxx or 10.0.xxx.xxx. Each jail also has an IP address, which needs to be different from anything else on your network. You can specify the range of IP addresses that will be assigned for jails in the web GUI, under Jails -> Configuration -> Advanced Mode. This range of IP addresses can't conflict with anything else on your network. For example, my FreeNAS server is at 192.168.1.10, its IPMI interface is at 192.168.1.11, and the IP range I've set up for my jails is 192.168.1.12-19. Alternatively, you can set your jails to use DHCP to get their IP addresses and other related configuration. I don't recommend this, as the IP addresses can change over time--but if everything on your network is getting its IP address from the same DHCP server, it's unlikely that you'll have IP address conflicts.
You also, most likely, have some sort of router that gives out IP addresses via DHCP. It's going to give out addresses in a certain range, which you can specify in the router's configuration software. You need to make sure that that range doesn't overlap with your jail IP range. Otherwise, you can have two things on your network with the same IP address, and there lies confusion.
Finally, you have the port number. The standard port for HTTP is 80, but for whatever reason, almost all of these web-based apps use something different, so you must specify the port when you try to access the app. The documentation should tell you what port is in use.
So, to put all of that together:
- The plugin must be installed
- The plugin must be enabled (Plugins -> Installed -> make sure that Service Status is On for the plugin in question)
- The Jail's IP address must not conflict with anything else on your network (see the Jails screen, and note the IP address of the jail you're interested in)
- You should then be able to reach http://jailip:portnumber with your web browser
If the first three conditions are met, and the fourth isn't, post in the Plugins forum describing what's happening.