Please visit this page here and take a look at your account balance, does it show up in points or currency (this cannot be undone)?
If the balance is showing as currency then it means the following:
A.) You or someone that uses your console has registered the console for the Xbox 360 Dashboard Beta back in July. This can be seen with the "Beta" channel on the dashboard. If you don't have this "Beta" channel, then the console you're on does not have the dashboard beta.
B.) You've downloaded your profile to a console that has the beta and your account has been converted from using Microsoft Points to your local currency.
If it's option B (sounds like it in your case), then this means that you can only make purchases on the console that has the beta dashboard. This dashboard will be released to everyone later this year (Fall 2013 - no other specific date just yet). Once you've made any sort of purchases you can then return back to your console and re-download that content using your download history (Dashboard > Settings > Account > Download History).