This is easily done...
1) Locate the script file "javakeys.js". If you installed AbelCam in the default location it's at C:\program files\abelcam\wwwroot\include\javakeys.js
2) Open the file in a text editor e.g. Notepad.
3) Find the line that looks like this:
code += "<INPUT type='checkbox' name='usekeys'>";
and change it to look like this:
code += "<INPUT type='checkbox' name='usekeys' CHECKED>";
4) Save the file. Reload the page in the browser. That should do it.
5) Important: Once you are satisfied with the changes save a copy of the file somewhere that you can find it again later. The /include folder gets replaced when you run the AbelCam installer or updater! You will need to copy your changed file back into /include after every program upgrade. If this seems like too much hassle, the alternative is to rename wwwroot to something else and then use Config->Web Server in AbelCam to point it at the new name. If wwwroot is not called wwwroot, the installer won't touch it or anything under it.