I cannot get wireless internet and every time I try to install it tells me there was an error and I have no demuxer plugin what can I do to fix this.

I assume you are using Ubuntu from the user agent icon in your post. It would be useful to know the make/model of your computer, also.

The crucial piece of information to get started, though, is the make and version of the wireless chipset. In a terminal, run the command lspci in a terminal (it stand for list pci devices), then post what it reports about your wireless back here; it is almost certain someone can help you.

Edit: Demuxer plugin for what precisely?

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I am using Inspiron 1545 this is what the lspci came up with

robin@robin-Inspiron-1545:~$ Lspci
No command 'Lspci' found, did you mean:
Command 'lspci' from package 'pciutils' (main)
Lspci: command not found
robin@robin-Inspiron-1545:~$
robin@robin-Inspiron-1545:~$ lspci
00:00.0 Host bridge: Intel Corporation Mobile 4 Series Chipset Memory Controller Hub (rev 07)
00:02.0 VGA compatible controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)
00:02.1 Display controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)
00:1a.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4 (rev 03)
00:1a.1 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #5 (rev 03)
00:1a.2 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #6 (rev 03)
00:1a.7 USB Controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #2 (rev 03)
00:1b.0 Audio device: Intel Corporation 82801I (ICH9 Family) HD Audio Controller (rev 03)
00:1c.0 PCI bridge: Intel Corporation 82801I (ICH9 Family) PCI Express Port 1 (rev 03)
00:1c.1 PCI bridge: Intel Corporation 82801I (ICH9 Family) PCI Express Port 2 (rev 03)
00:1c.2 PCI bridge: Intel Corporation 82801I (ICH9 Family) PCI Express Port 3 (rev 03)
00:1c.4 PCI bridge: Intel Corporation 82801I (ICH9 Family) PCI Express Port 5 (rev 03)
00:1d.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1 (rev 03)
00:1d.1 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2 (rev 03)
00:1d.2 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #3 (rev 03)
00:1d.7 USB Controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1 (rev 03)
00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev 93)
00:1f.0 ISA bridge: Intel Corporation ICH9M LPC Interface Controller (rev 03)
00:1f.2 SATA controller: Intel Corporation ICH9M/M-E SATA AHCI Controller (rev 03)
00:1f.3 SMBus: Intel Corporation 82801I (ICH9 Family) SMBus Controller (rev 03)
09:00.0 Ethernet controller: Marvell Technology Group Ltd. 88E8040 PCI-E Fast Ethernet Controller (rev 13)
0c:00.0 Network controller: Broadcom Corporation BCM4312 802.11b/g LP-PHY (rev 01)

I suspected it was Broadcom. Broadcom wireless usually requires an extra step or two to get working. This link will help:

https://help.ubuntu.com/community/Wi...Driver/bcm43xx

Broadcom drivers can be found in the "Proprietary drivers for devices (restricted)" repositories. In your package manager, enable them.

Also, look for an item in the Control Center under Hardware for "Additional Drivers for Devices." Either one of those avenues is likely to help.