How do I increase available disk space?

Usually by deleting/uninstalling programs etc. in your OS.

Now, what kind of error message did you get? A first post with extremely minimal information isn't really useful.

So, did you get a message like:
Message from server: (there was work but you don't have enough disk space allocated) ??
or
Message from server: No disk space (you must free '(number)' MB before BOINC gets space). review preferences for maximum disk space used. ??
or
Message from server: No disk space (you must free '(number)' MB before BOINC gets space). Review preferences for minimum disk free space allowed. ??

(Those are all links to articles in the BOINC Wiki)

In any case, you can set up all your preferences in the General preferences under Your account.

Look for the "Disk and memory usage" options and changes those accordingly to what is asked. Or better yet, give details on your system, including your harddrive space and free space and what the exact message is that BOINC throws at you. You can copy text from the Messages tab, by selecting the message(s) and then pressing the "copy selected messages" button on the side. They will be in your clipboard then, so you can post them here (rightclick, paste. Or CTRL+V)