By default the OpenWrt IP is 192.168.1.1. If you changed it you should know to what value did you change.
When accessing the router be sure that your PC or MAC is only connected with the MR3040 (best use a network cable).
For MAC I don't know but on PC go to 'Control Panel->Network and Sharing Center'.
Then press 'Change adapter settings' on left.
Then find your network adapter there, something like 'Local area connection'.
Right click on it and select 'Properties'.
In dialog select 'Internet protocol version 4 (TCP/IPv4)' and press the 'Properties' button.
In the dialog select 'Obtain an IP address automatically' and 'Obtain DNS server address automatically'.
Press the 'Ok' button to confirm.
Press the 'Ok' button again to close the dialog.
Now connect the MR3040 with your PC (network cable) and if not turned on then turn it on.
Wait a bit that windows get the IP address with DHCP.
Now right click again your network interface 'Local are connection' and select 'Status'.
In dialog press the 'Details' button.
In the upcoming dialog the 'IPv4 default gateway' should be the MR3040 address.
In my case it is '192.168.1.1'.
Now to access the MR3040 OpenWrt web interface I open a web browser and enter
http://192.168.1.1