I scoured the web looking for this information before finally figuring the solution out myself. The problem I was running into was I had no clue how to transfer a file to my iPhone without the phone being Jail broken, but since this app was bought at the official app store, I figured there had to be a way to do it via iTunes as the app suggests but sharing a file with an app is not something I had previously done, so here it is, a quick and simple guide to getting this done. This was done using Windows 7 and an iPhone 4.
Step 1: Start Mumble on the device your certificate (the one you want to use) is stored.
Step 2: On the menu choose Configure, then Certificate Wizard.
Step 3: Select "Export current certificate", then click Next.
Step 4: Click "Save As", this will open a browser window where you can select the name and where you want to put your certificate. Choose a file location and choose a file name that you can remember for later (I put mine on the desktop and named it MumbleCert), then click "Save".
Step 5: Click Finish. This will save the Mumble certificate where you chose with the as the name you chose.
Step 6: Connect your iPhone to your computer and start iTunes.
Step 7: On the left menu select your device, and sync it (right click your iPhone's name and select sync or click the sync button on the bottom right of the iTunes window) if you haven't done so since you installed the Mumble App on your iPhone.
Step 8: At the top of the main iTunes window where it displays the information for your phone, you have a number of options along the top; Summary, Info, Apps, Tones, etc... Select the Apps tab.
Step 9: Scroll down to the File Sharing section of the Apps window. You will see two smaller sections, Apps on the left, and Documents on the right. In the Apps section select Mumble, then select "Add..." on the Documents section.
Step 10: Browse to the Mumble Certificate you saved earlier, and choose "Open".
Step 11: On your iPhone open the Mumble App, select Preferences, then Certificate, Press the + button in the top right then select "Import from iTunes" then select the certificate.
You can now connect to your mumble servers using the certificate you moved from your computer.